The words googol (10 to the power of 100, or 1 with 100 zeroes after it) and googolplex were invented by a nine year old, who defined a googolplex as 1 followed by "writing zeroes until you get tired". His uncle, mathematician Edward Kasner, took all the fun out of it by then defining googolplex as 10 to power of 100 to the power of 100 (or put another way, googol to the power of 100). In so doing, he rendered his nephew's definition not merely obsolete, but possibly the greatest understatement in human history. If you wrote a googolplex via the zeroes method, you'd run out of atoms (roughly calculated at 10 to the power of 80) and even available space in the universe long before you got to the last zero.
The reciprocal of a googolplex (that is, 1 divided by a googolplex) is a googolminex. Dubious variants on these words are zootzootplex (coined by a precocious and no doubt highly irritating four year old), which is the exponential factorial of the googolplex (don't ask), and googolmilliplex which I'm not even going to try to define. The study of mathematical variants of googol has been dubbed googology. You'd think googolology would be more natural, but who knows what goes on in the heads of mathematicians?
Google, a search engine company you may have heard of, calls its world headquarters The Googleplex. Nice touch.