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

object-oriented polymorphism

    The kind of polymorphism found in object-oriented
   programming languages where a variable can refer to an object
   whose class is not known exactly until run time.  A method
   can use a variable of a given class - call other methods on it,
   pass it as an argument, etc. - without needing to know to which
   subclass it refers, as long as its actual class is compatible with
   those uses.

   (2014-01-05)