> I challenge you to find a definition for FP that includes Clojure, Erlang, F#, and OCaml but excludes Java, JS, Ruby, Python, C++
Challenge accepted: FP languages emphasise restriction of mutation.
Challenge accepted: FP languages emphasise restriction of mutation.
Favoring immutability is a great property, but it is not a hard dividing line. The Java community started encouraging immutability no later than 2001, and Scheme was called functional long before anyone categorized its main features as "restricting mutation".
(I seriously believe that any attempt to define FP will end in tears. It's a marketing term and has no real likelihood of producing legitimate technical value.)
Functional languages are languages used predominantly by functional programmers.
Being functional probably has more to do with how a language is used than the set of features it boasts.
Can you do FP in any language? Maybe not COBOL, but you could do it in C with function pointers. But it's still easier to do it in Haskell than in C, and it's still easier to do imperative programming in C than in Haskell.
So you have a point. But languages have different characteristics, too, and those differences let us classify languages, if not as "FP" and "non-FP", at least as "more FP" and "less FP".
FP as a programming style to which a programmer may adhere to a greater or less degree is easy to define. FP as a binary is-or-is-not characteristic of programming languages is, well, a misguided concept. To the extent it makes sense to talk about FP -- or OOP, or any other style -- as a feature of a programming language, it is more in terms of the degree to, or better the particular manner in, which it facilitates that style of programming, rather than a binary descriptor.