Container

The file wrapper holding video, audio and data together. Not the codec inside it; the box, not the contents.

The container is the shipping crate: video track, audio track, captions and metadata packed into one file with a manifest of what sits where. MP4, MOV, MKV are crates, not contents; the codec inside determines whether a player can actually decode what it finds.

The distinction earns its keep in every troubleshooting hour: a file may fail because the crate is unfamiliar or because the contents are, and the fixes differ, rewrapping is lossless and instant, re-encoding is neither. Knowing which problem you have is the difference between a minute and an afternoon.