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Definition
A container format (or wrapper) is a file format that holds multiple data streams — video, audio, subtitles, and metadata — in a single file. Common containers include MP4, MKV, WebM, AVI, and MOV. The container is separate from the codecs used to encode the media streams inside.
A container format is like an envelope that holds multiple types of data together. An MP4 file, for example, might contain an H.264 video stream, an AAC audio stream, subtitle tracks, chapter markers, and metadata — all synchronized and packaged in one file. The container handles timing, synchronization, and stream organization.
Common container formats include: MP4/M4V (most universal, supports H.264/H.265/AV1 + AAC/AC3), MKV (Matroska — very flexible, supports almost any codec combination), WebM (Google's web format, VP8/VP9/AV1 + Vorbis/Opus), AVI (legacy Microsoft format), and MOV (Apple QuickTime format).
Understanding the distinction between containers and codecs is important for compatibility troubleshooting. A file might be in an MP4 container but use a codec your device doesn't support (e.g., MP4 with AV1 on an older device). Conversely, the same video codec can be packaged in different containers — H.264 video works in MP4, MKV, MOV, and AVI containers. Transmuxing (changing containers without re-encoding) is fast and lossless when the target container supports the source codecs.