The Free On-line Dictionary of Computing (30 December 2018):

function application

   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).