A googol is a one with a hundred zeroes after it. That is: 10 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000 000. 

It's much overshadowed by its big brother, the GoogolPlex.

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See: GoogolPlex, BigNumbers