Hyperoperation

A hyperoperation is a generalization of addition, multiplication, exponentiation, tetration, etc. They are often written using Knuth's up-arrow notation. Natural number level hyperoperations can be defined recursively as a piecewise function:

$H_{n}(a,b)={\begin{cases}b+1&{\text{if }}n=0\\a&{\text{if }}n=1,b=0\\0&{\text{if }}n=2,b=0\\1&{\text{if }}n\geq 3,b=0\\H_{n-1}(a,H_{n}(a,b-1))&{\text{otherwise}}\end{cases}}\,\!$

This is used to express really large numbers while being able to write them.