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

functional requirements

    What a system should be able to do, the
   functions it should perform.

   This term is used at both the user requirements analysis and
   software requirements specifications phases in the software
   life-cycle.

   An example of a non-functional requirement is an
   initialisation sequence incorporated into the software that is
   specific to a given customer.

   (2001-05-22)