Here are some forms of polymorphism, from Types and Polymorphism in Persistent Programming Systems by R.C.H. Connor:
ad hoc: An operation is defined over a number of different types, and its semantics may depend upon the type of its operands. An example is the operator "+", which may be defined over integers and reals, and has a different interpretation for each type.He further writes:
parametric: Instances of the same type within a type description may be abstracted over by an implicit or explicit type parameter. An example is an identity function, where although the parameter and result may be of any type, they are statically known to be the same type.
inclusion: The type of a value may be partially abstracted over, so that unnecessary type information need not be stated. An example is a function which is defined over any record value which has an age field of type integer.
All of these language concepts have evolved independently from each other. Languages such as early assemblers contained no polymorphism whatsoever. Ad hoc polymorphism was the first to appear, in languages such as Fortran, which defined overloaded operators, such as "+", along with the ability to coerce values from integer to real according to the use of such operators. Parametric polymorphism appeared in ML, and inclusion polymorphism in Simula. Existentially quantified types as described by Mitchell and Plotkin is a model of abstract data types which introduces abstraction over a type description, and such types are included in the definition of parametric polymorphism given above.
The motivation for all of these diverse language models is the same: the need to abstract over type. As type systems include more static constraints, then flexibility is lost as well as safety gained. Some of this flexibility, however, may be regained without the loss of safety by the introduction of type abstraction. Polymorphism is viewed here not as some theoretical attribute of type systems but as a solution to a class of practical problems which require type abstraction.