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

abstract class

    In object-oriented programming, a class
   designed only as a parent from which sub-classes may be
   derived, but which is not itself suitable for instantiation.
   Often used to "abstract out" incomplete sets of features which
   may then be shared by a group of sibling sub-classes which add
   different variations of the missing pieces.

   (1994-11-08)