Feferman-Schütte ordinal

In mathematics, the Feferman–Schütte ordinal Γ₀ is a large countable ordinal. It is the proof theoretic ordinal of several mathematical theories, such as arithmetical transfinite recursion. It is named after Solomon Feferman and Kurt Schütte.

It is sometimes said to be the first impredicative ordinal, though this is controversial, partly because there is no generally accepted precise definition of "predicative". Sometimes an ordinal is said to be predicative if it is less than Γ₀.

Unfortunately there is no standard notation for ordinals at and beyond the Feferman–Schütte ordinal, so there are several ways of representing it, some of which use ordinal collapsing functions: ${\displaystyle \psi (\Omega ^{\Omega })}$, ${\displaystyle \theta (\Omega )}$ or ${\displaystyle \phi _{\Omega }(0)}$

...
Wikipedia