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An LLM (large language model) is a neural network that generates text by predicting the next token from the context it receives. Most current LLMs use Transformer architecture. For developers, the important consequences are practical: tokens and context windows limit requests, embeddings make semantic retrieval possible, prompting changes runtime instructions, fine-tuning changes model parameters, and retrieval-augmented generation (RAG) supplies external information at runtime. Reliable applications also need evaluations, access controls, and defenses against prompt injection.

This glossary explains those terms in the order they affect an AI application: how a model represents text, how it receives context, how you adapt it, and how you measure and secure the result.

What is an LLM and how does it generate text?

A large language model is a text-focused foundation model trained on a very large corpus. Its learned weights encode statistical and semantic patterns. During generation, the model predicts a probability distribution for the next token, selects a token according to its decoding settings, adds it to the context, and repeats the process until it reaches a stopping condition or output limit.

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Generative AI

Generative AI is the broader category of systems that create new content, including text, images, audio, code, or video. An LLM is the language-and-text member of that family; calling every generative model an LLM is inaccurate.

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Transformer

A Transformer is the dominant neural-network architecture for modern language models. Its self-attention mechanism lets each token weigh information from other tokens in the supplied sequence, helping the model connect words and concepts that are far apart. Implementations differ: do not assume every production model has the same encoder, decoder, or attention layout.

Parameters, weights, and pretraining

A parameter (also called a weight) is a learned numerical value. The collection of weights stores patterns acquired during training. Pretraining is the initial, usually large-scale training phase; an autoregressive model commonly learns by predicting the next token from preceding tokens, even when the source corpus has no human labels.

Inference

Inference means running the trained model on an input to produce a prediction or generated response. In an application, inference is the serving-time operation, separate from pretraining or fine-tuning. Its latency and cost depend on the model, input and output lengths, hardware, batching, and service configuration.

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What is a token, and why does a context window matter?

A token is the unit a model processes. Depending on the tokenizer, a token may represent a character, a whole word, part of a word, punctuation, or whitespace. Tokenization converts text into the model’s vocabulary items and token IDs; the vocabulary is the set of IDs that tokenizer can emit.

The same sentence can produce different token counts on different models because each model can use a different vocabulary and tokenizer. Token counts affect request limits, usage accounting, prompt design, and latency, so measure with the tokenizer for the model you actually call rather than estimating from characters or words.

Context window

A context window is the amount of tokenized text a model can look back at and process for one request. It is working memory for that request, not the model’s complete training corpus and not automatically a permanent memory store. The prompt, conversation history, retrieved documents, tool results, and requested output all consume model-specific limits; verify current input and output limits in the documentation for the chosen model.

Design implications

  • Count tokens before sending large documents or long conversation histories.
  • Reserve space for the answer when setting an output limit.
  • Trim, summarize, or retrieve only relevant history when a context window is nearly full.
  • Expect longer contexts to affect latency and potentially cost, even when they fit the formal limit.

What are embeddings and vector search used for?

An embedding is a vector representation of text or another input, designed so useful relationships are reflected in geometric distance. Similar meanings tend to produce nearby vectors, although “nearby” is meaningful only for the embedding model and similarity method being used.

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Embeddings support semantic search, clustering, recommendations, anomaly detection, and classification. In a retrieval system, an application embeds documents, stores those vectors, embeds a user query, and searches for nearby document vectors.

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Vector database and similarity search

A vector database stores embedding vectors and returns nearby vectors for a query vector. It is an implementation component, not the embedding model itself. A production search may combine vector similarity with keyword filters, metadata filters, permissions, recency, or a reranking step.

Embedding quality, chunk boundaries, metadata, and access filtering determine what a model can retrieve. A sophisticated generator cannot answer from a document that the retriever failed to return.

How do prompting and instruction layers work?

A prompt is the input supplied at inference time: instructions, examples, user data, conversation state, and sometimes retrieved documents or tool results. Prompts are not the model’s training data and do not alter its weights.

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System, developer, and user instructions

Many APIs expose separate system, developer, and user messages. These layers commonly express application-wide behavior, implementation constraints, and the user’s request, respectively. Precedence and conflict handling are defined by the particular API or framework; do not assume all vendors implement identical roles. Enforce critical permissions in application code rather than relying on a message hierarchy.

Prompt engineering

Prompt engineering is the deliberate design and iteration of inputs to obtain a desired result. A robust prompt specifies the task, relevant context, constraints, success criteria, and output format. Examples can show edge cases and formatting more reliably than a vague request.

  • Zero-shot prompting: asks for the task without an example.
  • One-shot prompting: includes one example.
  • Few-shot prompting: includes several examples and is a form of in-context learning.

Prompt tuning is not prompt writing

Prompt tuning is a parameter-efficient adaptation method that learns a soft prompt or prefix while leaving most model parameters frozen. Ordinary prompt engineering edits human-readable text at runtime; prompt tuning trains learned vectors.

Temperature and context engineering

Temperature is a decoding control that generally makes outputs more varied at higher values and more deterministic at lower values. Exact behavior is implementation-specific, and temperature does not make an incorrect answer factual. Inference-time context engineering is the broader practice of selecting and ordering instructions, user data, retrieved passages, conversation state, and tool results so the model receives the right information in the right form.

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What is RAG, and how does grounding reduce hallucinations?

Retrieval-augmented generation (RAG) is a three-stage pattern: retrieve relevant data, augment the model prompt with that data, and generate a response. It can provide current, proprietary, or domain-specific information that was absent from pretraining without changing model weights.

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  1. Index: split authorized source material, create embeddings, and store vectors with metadata.
  2. Retrieve: embed the query and return relevant passages, applying permission and metadata filters.
  3. Augment: place the selected passages and their source information in the model’s context.
  4. Generate: instruct the model to answer from the supplied evidence and to acknowledge when evidence is insufficient.

Grounding connects an answer to trusted or retrieved evidence and can support source attribution. It improves reliability but is not a correctness guarantee: retrieval can miss the right passage, a source can be wrong, and the model can misread or overstate evidence.

Hallucination

A hallucination is an output that sounds plausible but is unsupported or factually wrong. RAG, citations, constrained formats, and verification checks reduce this failure mode; none proves that every generated statement is true.

Tool use, function calling, and agents

Tool use or function calling lets a model request that application code or an external service perform a structured operation. Validate arguments, authenticate the caller, authorize the action, and check the result in code; model-generated text is not a security boundary.

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An AI agent is the surrounding application that combines a model with orchestration, state or memory, and tools to pursue a goal. The agent is not merely the underlying model. More tools and autonomy increase the need for limits, logging, and human approval for consequential actions.

What is the difference between prompting, RAG, and fine-tuning?

Choose among them by asking where the needed knowledge lives and how often it changes. Prompting changes the request, RAG changes the runtime evidence, and fine-tuning changes the model’s learned parameters.

Approach Where knowledge or behavior lives Update path Main operational concerns What to evaluate
Prompting Instructions and examples in the context Edit the prompt or message templates Context length, instruction conflicts, and runtime cost Instruction adherence and task quality
RAG External documents retrieved at runtime Refresh documents, embeddings, and the index Retrieval latency and recall, permissions, citations, and prompt injection in retrieved text Retrieval recall, groundedness, citation quality, and answer quality
Fine-tuning Model parameters or adapter weights Train an adapted model or adapter on new examples Training data quality, compute, versioning, and possible data leakage Task metrics, instruction adherence, safety, and regression against a baseline

Use RAG when facts are private, changing, or need source attribution. Use fine-tuning when repeated examples show a stable task, style, or behavior that prompts cannot reliably produce. The approaches can be combined: a tuned model can still use RAG, and a carefully designed prompt remains necessary for either.

When should I fine-tune, and what do PEFT and LoRA mean?

Fine-tuning continues training a pretrained model on additional examples so it learns task, domain, or style patterns. It changes model parameters, unlike supplying an instruction in a prompt.

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Instruction tuning

Instruction tuning is fine-tuning on instruction-and-response examples to improve the model’s ability to follow natural-language tasks. Quality and coverage of examples matter more than simply increasing their count.

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Parameter-efficient fine-tuning (PEFT)

PEFT trains only a small subset of parameters or an adapter while keeping most base weights frozen. This reduces compute and storage requirements compared with updating every weight.

LoRA

Low-Rank Adaptation (LoRA) is a PEFT method that trains low-rank update matrices while the base model remains largely frozen. Serving systems can attach multiple adapters to one base model, but each adapter still needs its own evaluation and version controls.

Distillation

Distillation trains a smaller student model to reproduce useful behavior from a larger teacher model. The exact loss function and data recipe vary by implementation, so validate the student on the target workload instead of assuming that teacher-like outputs guarantee equivalent quality.

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Quantization and caching

Quantization represents weights or activations with lower numerical precision to reduce memory use and improve serving efficiency, with possible quality trade-offs. Benchmark the selected implementation on representative inputs. Caching reuses previously computed prompts, embeddings, or responses to reduce latency and cost; define cache keys, freshness rules, and privacy boundaries explicitly.

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How do I evaluate an AI application?

An LLM evaluation, or eval, is a set of tests, metrics, or benchmarks for task quality, helpfulness, safety, and other requirements. Build it around the real user outcome rather than relying only on a generic benchmark score.

Prompt and response sets

A prompt set is the collection of evaluation inputs. A response set is the model’s output for those inputs. Version both sets so a prompt, model, retrieval, or code change can be compared against a known baseline.

Human evaluation

People can review correctness, relevance, style, or safety using a rubric. Record the rubric, sampling method, evaluator instructions, and disagreements; there is no universal human-rating protocol that fits every application.

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Latency and throughput

Latency is the time to produce a response, including time to first token in a streaming system. Throughput is the amount of work completed per unit time. Measure both under representative concurrency, prompt lengths, retrieval behavior, and tool calls rather than quoting a single best-case run.

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Provenance

Provenance is information about how digital content was created or changed. Retain the document versions, model and adapter versions, prompt template, retrieved sources, tool calls, evaluator results, and relevant timestamps needed to audit an output.

What security terms should every LLM developer know?

Prompt injection

Prompt injection is crafted input that attempts to override intended instructions or manipulate model behavior. Treat user text, retrieved documents, web pages, and tool outputs as untrusted input. Keep secrets out of model-visible context where possible, isolate instructions from data, validate tool arguments, enforce permissions outside the model, and constrain outputs before executing them.

RLHF and preference optimization

Reinforcement learning from human feedback (RLHF) and other preference-optimization methods use human or preference signals to shape model behavior after pretraining. Algorithms, data, and guarantees vary by model, so the acronym alone does not establish a particular safety or quality level.

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HHH

HHH describes three desired properties: helpful assistance, honest behavior that acknowledges limitations, and harmless or non-harmful assistance. It is a behavioral framework, not a proof that a system is safe in every context.

How do these terms fit together in a production system?

  1. Tokenize and budget the request so instructions, retrieved evidence, history, and output fit the context window.
  2. Use clear system or developer instructions and representative examples, then choose decoding settings appropriate to the task.
  3. For changing or private facts, retrieve authorized passages through embeddings and a vector search layer.
  4. Pass tool requests through typed schemas, authentication, authorization, validation, and logging.
  5. Run versioned evals for quality, groundedness, safety, latency, and throughput before and after every material change.
  6. Store provenance and monitor failures such as unsupported claims, retrieval misses, prompt injection, and stale caches.

Frequently Asked Questions

Can a longer context window replace a database or memory system?

No. A context window is temporary working memory for a request, and older conversation content may need to be truncated or summarized. Durable memory requires an application-managed store with explicit retention, retrieval, and deletion rules.

How should I test an LLM feature when there is no single correct wording?

Define acceptance criteria for the outcome, such as required facts, prohibited claims, format, and safety behavior. Combine automated checks with rubric-based human review, and keep difficult edge cases in a versioned regression set.

Should retrieved documents be trusted because they came from my own system?

No. Internal documents can be stale, incorrectly permissioned, or contain instructions aimed at manipulating the model. Apply access controls and treat every retrieved passage as data to be evaluated, not as an instruction with authority.

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What should I log when a model calls a tool?

Record the model and prompt versions, structured arguments, authorization decision, tool result, timestamps, and the final response. Redact secrets and personal data according to your retention policy, and make logs tamper-evident when actions have regulatory or financial consequences.

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