>I think Polymorphism means that a function can do different things
depending
>on the data you give it.  A good example would be overloading + to do

Not exactly, that is operator overloading
Here is a definition
http://webopedia.internet.com/TERM/p/polymorphism.html

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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 circumference methods for any
number of derived classes, such as circles, rectangles and triangles.
No matter what shape an object is, applying the circumference method
to it will return the correct results. Polymorphism is considered to
be a requirement of any true object-oriented programming language
(OOPL).

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