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

subtype
subtyping

    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)