Xkcd number

The xkcd number is equal to $$A(G, G)$$, where G is Graham's number and A is the Ackermann function. It was invented by Randall Munroe, the creator of the popular webcomic xkcd, in strip #207, which says, "[xkcd] means calling the Ackermann function with Graham's number as the arguments just to horrify mathematicians." The number was given its name by various other bloggers.