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

formal methods

    Mathematically based techniques
   for the specification, development and verification of
   software and hardware systems.

   Referentially transparent languages are amenable to symbolic
   manipulation allowing program transformation (e.g. changing
   a clear inefficient specification into an obscure but
   efficient program) and proof of correctness.

   Oxford FM archive
   (http://comlab.ox.ac.uk/archive/formal-methods.html).

   (1996-05-15)