even though here u will find lotz of techies who
will answer ur question within seconds.. just google
it "Polymorphism" u will find more world class
definitions.. cuz polymorphism is a very common word
OOPS

follwing lines are taken from
http://www.webopedia.com/TERM/p/polymorphism.html

Polymorphism
============
Generally, the ability to appear in many forms. In
object-oriented programming, polymorphism refers to a
programming language's ability to process objects
differently depending on their data type or class.
More specifically, it is the ability to redefine
methods for derived classes. For example, given a base
class shape, polymorphism enables the programmer to
define different area methods for any number of
derived classes, such as circles, rectangles and
triangles. No matter what shape an object is, applying
the area method to it will return the correct results.
Polymorphism is considered to be a requirement of any
true object-oriented programming language (OOPL).