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

functional specification

    A description of what a system (e.g. a
   piece of software) does or should do (but not how it should
   do it).  The functional specification is one of the inputs to
   the design process.

   See IEEE/ANSI Std. 610.12-1990.

   (1999-04-07)