The term "protocol" is held over from Obj-C. I guess because in Obj-C, the declaration of a class is wrapped in @interface...@end (the implementation is inside @implementation...@end)

And it's annoying to have to explain this to all the other devs on the team who don't use Obj-C. Here's hoping Swift fixes the vocabulary.

EDIT: I suppose the main difference between Swift's "protocol" and Java's "interface" is that Swift lets the dev provide actual implementations for some declared functions. "Protocol" is still not an idea word in my opinion.