Source : Free On-Line Dictionary of Computing

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)