No, it refers to functions f where forall x, f(f(x)) = f(x). You can abuse notation slightly to see this as f^2(x) = f(x). (This kind of notation is actually u

No, it refers to functions f where forall x, f(f(x)) = f(x). You can abuse notation slightly to see this as f^2(x) = f(x). (This kind of notation is actually used enough to be comfortable to most mathematicians.) From here it's not much of a stretch to eta-contract this to f^2 = f. And so we've ended up back in the same place...