From Nico Nico Douga
http://www.nicovideo.jp/watch/sm17427094
Author: Powatsu
Release: 01/04/2012
In mathematics, tetration (or hyper-4) is the next hyper operator after exponentiation, and is defined as iterated exponentiation. The word was coined by Reuben Louis Goodstein, from tetra- (four) and iteration. Tetration is used for the notation of very large numbers.