A function applied to (some of) its arguments. If it is not applied to all its argument then it is a "partial application". Application is usually written in the form f(x) but some languages such as command-line interpreters and many functional languages use juxtaposition: f x. Lisp places the parentheses around the whole application: (f x).
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Nearby terms: functional specification « functional testing « functional unit « function application » function complete » Function Graph Language » function inlining