WordNet (r) 3.0 (2006):

supercomputer
    n 1: a mainframe computer that is one of the most powerful
         available at a given time

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

supercomputer

    A broad term for one of the fastest computers
   currently available.  Such computers are typically used for
   number crunching including scientific simulations,
   (animated) graphics, analysis of geological data (e.g. in
   petrochemical prospecting), structural analysis, computational
   fluid dynamics, physics, chemistry, electronic design, nuclear
   energy research and meteorology.  Perhaps the best known
   supercomputer manufacturer is Cray Research.

   A less serious definition, reported from about 1990 at The
   University Of New South Wales states that a supercomputer is
   any computer that can outperform IBM's current fastest, thus
   making it impossible for IBM to ever produce a supercomputer.

   (1996-12-13)