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Lossy file compression makes a file smaller by discarding some of its information. When the compressed file is decoded, the result is not exactly the original. The differences may be hard to see or hear, but “hard to notice” is a different property from “identical to the original.” That gap is the whole definition.
The formal definition
ITU-T Recommendation T.800, clause 3.62 (2015 edition), defines the term this way: “A descriptive term for the effect of the overall encoding and decoding processes in which the output of the decoding process is not identical to the input to the encoding process.” The same clause adds two conditions that make the idea concrete: “There is distortion (measured mathematically). At least one of the coding processes or steps used for encoding and decoding is irreversible.”
Two things follow from that wording. First, the test is mathematical: the decoded output does not match the input. Second, the loss is built into the process. At least one step cannot be undone, so no decoder can rebuild the discarded information.
Lossy versus lossless
Lossless compression is the opposite case. The same standard describes lossless coding as one where the decoded output is identical to the encoding input, so the original can be restored exactly. The table below sets the two side by side.
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| Question | Lossy compression | Lossless compression |
|---|---|---|
| Is the decoded output identical to the original? | No. Per ITU-T T.800, output is not identical to the input. | Yes. Decoded output is identical to the encoding input. |
| Does the process contain an irreversible step? | Yes, at least one. | No irreversible step is involved in restoring the data. |
| Can the original be recovered exactly? | Cannot be guaranteed. | Yes. |
| Typical file size outcome | Smaller, at the cost of exact fidelity. The size reduction for a given file is not stated by the source and depends on the format and settings. | Smaller than the original, but generally by less than lossy coding can achieve. Specific ratios are not stated by the source. |
Lossy is therefore a deliberate trade. You accept a file that differs from the source in exchange for a smaller representation. Whether that trade is acceptable depends on what the file is for.
Why “lower quality” is not the full definition
People often describe lossy compression as “lower quality.” That phrasing is loose. Under the formal definition, the defining fact is non-identity: the output is not the input. Whether a person notices the difference, or cares, is a separate question. A lossy file can look or sound indistinguishable from the source and still be lossy, because the discarded information is gone even if nobody can see or hear its absence.
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“Visually lossless” is still lossy
IETF RFC 8761 (2020) uses the phrase “visually lossless” in its terminology section. It describes a form of lossy compression in which the data lost after compression and decompression “is not detectable to the eye,” and the compressed data “appear identical to the uncompressed data.”
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A format family can support both modes
A file type’s name does not always tell you whether it is lossy. JPEG 2000 is the clearest example. According to the ITU-T T.800 record (V4, July 2024) and the ISO/IEC 15444-1:2024 core standard, JPEG 2000 defines both lossless and lossy still-image compression. Two JPEG 2000 files can therefore differ in whether they can be restored exactly, depending on the mode used to create them.
The practical rule: do not assume that a format family name settles the question. Check the mode or settings used to create the specific file.
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Deciding whether lossy compression fits
The definition does not tell you whether a lossy file is good enough. Four questions do.
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- Do you need exact recovery? If the original must be restored bit for bit, lossy coding is ruled out, including visually lossless modes.
- How much size reduction matters? Lossy coding is used to get smaller files. The sources do not provide universal size ratios, so test with your own material and settings.
- Are the discarded differences acceptable for the intended use? Decide this for the specific material. A difference that is invisible in a photo may matter in a scientific image or a legal record.
- Does your workflow support the format and mode? Confirm that the software and devices you rely on can open the lossy variant you produce.
If the answer to the first question is yes, a lossless format is the correct choice regardless of the other three.
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Recognising lossy output in practice
The compression mode is set when the file is created. Look for it in the export or save options of the software that made the file, and in the file’s technical details where that software exposes them. Do not rely on the file extension alone, because the same extension can cover lossy and lossless output, as the JPEG 2000 example shows.
Once a file has been saved in a lossy mode, the discarded information cannot be recovered from that file. Keep the original source if you may need it again, and save any later edits from the original rather than repeatedly re-saving the lossy copy.
Scope of this definition
This article defines the term and explains the distinctions the formal sources draw. It does not explain how any particular codec works internally, and it does not claim that every lossy method uses a given technique or that every lossy operation produces visible damage. Specific behaviour depends on the format, version and settings in use.
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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallSources cited: ITU-T Recommendation T.800 (2015 edition, clause 3.62, and V4, July 2024); ISO/IEC 15444-1:2024; IETF RFC 8761 (2020).
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