The Googolplexianth is the largest number of whatever…at least, the one that has been given an official name, yet isn't bigger than infinity. A Googolplexian is defined at 10Googolplex. A Googolplex is defined as 10Googol. A Googol is defined as 10100.
The name of it was invented by a 9-year-old boy, who was the nephew of the mathematician Edward Kasner. However, there is no such thing as the largest number, because as soon as we name it, there is always something bigger.