I think 55pts makes an important distinction: just because it's possible to write programs in a functional style doesn't mean it's a functional language. The answer with the most votes in your SO link sums it up: Ruby is an imperative, OO language, and you can be generous and add that it has first-class functions and lexical closures. But JavaScript, Python, Perl, PHP, and C# fall in this category, and maybe Java 8 will too. If they are all "functional languages" then the phrase is meaningless.