Glossary

Short definitions for terms used throughout this reference. Where a term has its own full page, the definition links there.

Bitrate: The amount of data used to represent a video or audio stream per second, usually in kilobits or megabits per second. See choosing a bitrate.
Codec: Short for "coder-decoder." A defined method for compressing raw video or audio into a smaller stream and decompressing it again for playback. Examples: H.264, H.265, AAC, Opus. See H.264 vs H.265.
Container: A file format that wraps together compressed video, audio, subtitles, and metadata, along with a table describing how they line up in time. Examples: MP4, MKV, WebM. See containers explained.
CBR (Constant Bitrate): An encoding mode that targets the same bitrate throughout the video regardless of scene complexity, prioritizing predictable data rate over efficiency.
VBR (Variable Bitrate): An encoding mode that varies bitrate over time, spending more data on complex scenes and less on simple ones, to hold a more consistent quality level.
CRF (Constant Rate Factor): An encoding mode where you specify a target quality rather than a bitrate; the encoder uses as much data as needed, scene by scene, to hit it. Common for offline, non-live encoding.
Chroma subsampling: A compression technique that stores color information at lower resolution than brightness (luma) information, exploiting the eye's lower sensitivity to color detail. Written as ratios like 4:2:0 or 4:4:4.
Frame rate: The number of frames displayed per second (fps). Common values include 24, 25, 30, and 60 fps. See resolutions & frame rates.
GOP (Group of Pictures): A sequence of frames starting with a keyframe, followed by frames that reference it and each other. GOP length affects both compression efficiency and how quickly a player can seek or switch quality levels.
HDR (High Dynamic Range): A way of encoding a wider range of brightness and color than standard ("SDR") video, aiming to more closely match what the eye can perceive in high-contrast scenes.
HLS (HTTP Live Streaming): An adaptive streaming format that describes available quality levels in an .m3u8 playlist and serves video as short segments over plain HTTP. See adaptive streaming.
DASH (Dynamic Adaptive Streaming over HTTP): An adaptive streaming format similar in purpose to HLS, describing renditions in an .mpd manifest. See adaptive streaming.
Keyframe / I-frame: A frame encoded as a complete, self-contained image, without referencing other frames. Playback and seeking can only start cleanly from a keyframe.
Lossy / lossless: Lossy compression discards some information permanently to save space (most video codecs); lossless compression preserves the original data exactly, at the cost of much larger file sizes.
Muxing / demuxing: Muxing combines separate compressed streams (video, audio, subtitles) into a single container file. Demuxing splits them back apart. See containers explained.
Manifest: A text file listing the available quality levels (renditions) for an adaptive stream and where to find their segments. See adaptive streaming.
Rendition: One specific quality level (a resolution and bitrate combination) of an adaptively streamed video.
Resolution: The number of pixels in a frame, expressed as width × height (for example, 1920×1080). See resolutions & frame rates.
Segment: A short chunk of video, typically a few seconds long, that an adaptive streaming player downloads and plays in sequence.
Transcoding: Converting video from one codec, bitrate, or resolution to another by fully decoding it and re-encoding the result, as opposed to remuxing, which repackages a stream without re-encoding it.