Source : Free On-Line Dictionary of Computing

subtype
     
         If S is a subtype of T then an expression of
        {type} S may be used anywhere that one of type T can and an
        {implicit type conversion} will be applied to convert it to
        type T.
     
        In {object oriented programming}, this means that {objects} of
        type S must accept every {message} that one of type T would.
     
        (1997-07-28)