Codec
A codec is the method used to compress audio or video for storage or sending, and to decompress it again for playback.
Also known as video codec, audio codec, coder-decoder
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The word joins coder and decoder. An encoder compresses the picture or sound, a decoder rebuilds it, and the codec is the agreed format between them. Nearly every codec used for streaming and calls is lossy: it throws away detail the eye or ear is unlikely to miss, which is how a 4K film fits through a home connection.
The video codecs a browser commonly plays are AV1, HEVC (H.265), VP9 and H.264 (AVC). Newer codecs reach the same quality at a lower bitrate, at the cost of more work to encode and decode. That work is why the question of whether a device decodes a codec in hardware matters: a codec the graphics chip decodes is cheap, while the same codec decoded on the processor can stutter and drain a battery. Bluetooth audio has its own codecs, such as SBC, AAC, aptX and LDAC, and the one the link negotiates affects both quality and delay.
A codec is not a file format. MP4 and WebM are containers that hold streams in one codec or another, so two MP4 files can need quite different decoders. A browser can say whether it supports a codec and whether it offers a hardware decoder for it; it is never told which Bluetooth codec a headset is using.
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