googol

10¹⁰⁰

That's 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

In 1920¹, the American mathematician Edward Kasner asked his nine-year-old nephew Milton Sirotta to help him come up with a name for a very large number, ten to the power of one hundred. The boy said to call it a googol. Kasner thought this was a number to overflow people's minds, being bigger than anything that can ever be put into words. Another mathematician then shot back with googolplex, and defined it to be 10 to the power of googol.

Googol and some of its friends:

• Avogadro's number: 6.0221415x10²³
• Little googol: 2¹⁰⁰
• The Universe: 10⁷⁹ up to 10⁸¹ particles.
• Googol: 10¹⁰⁰
• The Shannon number: 900⁴⁰, or about 10¹¹⁸.
• Archimedes' really big number: 10^8x1016 (as appearing in The Sand Reckoner)
• The almighty googolplex: 10^googol

Google.com is said to be named after this number.

Footnotes:
1. The exact date is not certain, but the other date you often see, 1938, is the date of a Time magazine article that helped bring the googol into common usage.

References:
http://www.informatik.uni-frankfurt.de/~fp/Tools/Googool.html
http://en.wikipedia.org/wiki/Googol
http://www.time.com/time/magazine/article/0,9171,931101,00.html
http://en.wikipedia.org/wiki/The_Sand_Reckoner