I'll link the wikipedia thread, but here is a short mini-definition:
The meaning of the term can range from the person who knows a lot about several fields of study or has an encyclopaedic knowledge to the person who has proficiency and competence in multiple fields or even to the person who has excelled in multiple fields.
Also to further summarize, a polymath is generally considered someone who is both a universal genius and a Renaissance man.

http://en.wikipedia.org/wiki/Polymath

So what would be the types of these noteable polymaths?

- Johann Wolfgang Goethe:

- Gottfried Leibniz:

- Leonardo Da Vinci:

- Aristotle:

- Benjamin Franklin:

- Thomas Jefferson:

- Blaise Pascal:

- Alexander von Humboldt:

- Leone Battista Alberti: