The Collaborative International Dictionary of English v.0.48:

Polymorphic \Pol`y*mor"phic\, a.
   Polymorphous.
   [1913 Webster]

The Free On-line Dictionary of Computing (17 December 2009):

polymorphism
parametric polymorphism
polymorphic

    A concept first identified by
   Christopher Strachey (1967) and developed by Hindley and
   Milner, allowing types such as list of anything.  E.g. in
   Haskell:

   	length :: [a] -> Int

   is a function which operates on a list of objects of any type,
   a (a is a type variable).  This is known as parametric
   polymorphism.  Polymorphic typing allows strong type checking
   as well as generic functions.  ML in 1976 was the first
   language with polymorphic typing.

   Ad-hoc polymorphism (better described as overloading) is the
   ability to use the same syntax for objects of different types,
   e.g. "+" for addition of reals and integers or "-" for unary
   negation or diadic subtraction.  Parametric polymorphism
   allows the same object code for a function to handle arguments
   of many types but overloading only reuses syntax and requires
   different code to handle different types.

   See also generic type variable.

   In object-oriented programming, the term is used to describe
   a variable that may refer to objects whose class is not
   known at compile time and which respond at run time
   according to the actual class of the object to which they
   refer.

   (2002-08-08)