Source : Free On-Line Dictionary of Computing

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)