Container format
The file format holding video, audio, subtitles and metadata together — but saying nothing about how the contents are encoded.
The distinction from codec is the commonest source of confusion. An .mp4 file can contain H.264, H.265 or AV1; the container only describes how the tracks are stacked and synchronised. A player opening the file but giving only audio nearly always means the codec is missing, not the container.
Packing tracks together is called muxing and taking them apart demuxing, and it can be done without touching the video itself — which is why changing container takes seconds while changing codec takes minutes. The choice is mostly driven by where the file will be played: MP4 works everywhere, MKV does more, WebM is the web's open alternative.