The difference between an API and a protocol is that a protocol calls itself a protocol.

A good protocol is documented and has multiple implementations. But an API can have that too. A protocol has to specifically by created with the purpose of there being multiple implementations to be a protocol.

ISO 3103 is a standardized method for brewing tea. Protocol.

Electrical outlets, light bulb screws, all protocols.

Snapple facts - API.

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I miss the good old glass Snapple ice teas

Good explanation. I would say an API can *become* a (quasi) protocol (see e.g. S3 API), but a protocol can not become just an API.

the same way many backends have successfully reimplemented mastodons web api, while very few manage to make activitypub work as an actual protocol, as a bad actor's implementation owns the majority of the network and refuses to follow the specification