A "googolplex" is an extremely large number expressed by 1 followed by a googol of zeros. A googol is mathematically expressed as 10 to the 100th power.
The words googol and googolplex first appeared in print in the 1940 edition of "Mathematics and the Imagination," co-written by Edward Kasner and James R. Newman. The book credits Kasner's nephew with coining the words as a response to Kasner's request to name an enormous number. As of 2014, it is impossible to express a googolplex in standard form. Doing so, explained astronomer Carl Sagan, would require more space than is available in the known universe.