I think I didn't express myself quite right. As mightybyte says in his comment on the successor thread to this one (https://news.ycombinator.com/item?id=1081219

I think I didn't express myself quite right. As mightybyte says in his comment on the successor thread to this one (https://news.ycombinator.com/item?id=10812198 https://news.ycombinator.com/item?id=10812198), functional programming is a programming paradigm that focuses on functions as the basic building blocks of a program. (As opposed to, say, objects in OOP.) Everything else that is part of FP flows from this - if you will, that is the essence of functional programming. However, "it has an emphasis on functions" is a rather wishy-washy definition for a functional programming language. Thus I touted first-class functions as a working definition for how to classify a language as functional. It's not the whole story, but it indicates an underlying philosophy. And of course, languages almost always fall somewhere on the continuum of the paradigm spectrum. Even many Lisp dialects have become multi-paradigm nowadays. "Functional" simply means that a language supports functional programming well.