Codec

The language a moving image is encoded and read in. Invisible when it works, and the reason a file will not open when it does not.

A codec is a shared agreement about how moving pictures become numbers and back: the encoder's dialect must be one the player speaks. Inside the agreement live decades of perceptual research, what to keep, what the eye forgives, which is why codecs age like languages, replaced by more efficient descendants.

Practically, codec choice is an audience question: the newest one saves half the weight and strands the oldest devices; the veteran plays everywhere at a size penalty. The web's answer is to offer both and let the browser choose, modern dialect first, universal fallback behind it, which is exactly how our video sources are ordered.