subtype

from The Free On-line Dictionary of Computing (8 July 2008)

subtype
subtyping

   <programming> 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)