Glossary

Terms used throughout this desk, defined in plain technical language. Where a term has a subtle implication, the note explains it rather than merely naming it.

Core terms

Macroblock: A fixed sixteen-by-sixteen sample region used by older codecs as the basic coding unit.
Coding tree unit: A quad-tree region in HEVC that subdivides into coding units of varying size.
Superblock: The AV1 analogue of a coding tree unit, up to one hundred and twenty-eight samples square.
Prediction: Forming a block from already-known data, either neighbours within a picture or a displaced region of another picture.
Residual: The difference between a prediction and the actual samples; this is what gets compressed.
Transform: A mathematical step that concentrates residual energy into few coefficients.
Quantisation: Controlled discarding of coefficient precision; the primary lever for compression.
Entropy coding: Lossless coding of symbols using their statistical behaviour.

Structure and control

GOP: Group of pictures; the span between key frames over which inter prediction operates.
Key frame: A picture coded without reference to another, providing a seek point.
Rate factor: A dimensionless quality dial that leaves the bitrate unconstrained so it follows scene complexity.
Preset: A named setting that trades encoding time for compression efficiency.
Profile: A defined subset of codec tools that a decoder must support.
Level: A bound on resolution, frame rate and bitrate combinations.

Delivery and measurement

Container: Storage and indexing format that holds elementary streams together.
Elementary stream: A single coded video or audio track before multiplexing.
Bitrate ladder: An ordered set of renditions for adaptive selection.
Signal metric: A measurement comparing reconstruction with source, such as peak signal-to-noise ratio.
Perceptual metric: A measurement calibrated against human ratings rather than raw error.

For context on how these terms combine, start at the overview or the standards page.