Call/cc

call/cc is pretty much the strangest language construct ever invented. It's common in LISP-like languages and is incredibly difficult to understand.

An attempt to put it in English (by someone who doesn't understand it)
call/cc takes a single function- f- as its argument. f takes a single argument- another function, called c- as its argument. call/cc calls f with a special function as an argument called the "current continuation". When the current continuation- basically, a single-argument function- is called with a value v, it resets the state-of-control to when the original call/cc was called and forces it to return the argument it was applied to. Phew!

Usage in Esolangs
call/cc basically makes any language that uses it an honorary esolang. However, this section will only cover intentional esolangs.