"Functional programming" is a useful description for techniques/approaches. The unifying principle tends to be (a) semantics based on definition and substitutio

"Functional programming" is a useful description for techniques/approaches. The unifying principle tends to be (a) semantics based on definition and substitution of values/expressions (e.g. no ambient state/context) and (b) 'direct' evaluation strategy for expressions (e.g. no search, like in logic programming). "Functional language" is less concrete; it's just an informal way to imply how easy or awkward it is to use these techniques in a particular language (e.g. bash is "less functional" than ML).