In mathematics, the Ackermann ordinal is a certain large countable ordinal, named after Wilhelm Ackermann. The term "Ackermann ordinal" is also occasionally used for the small Veblen ordinal, a somewhat larger ordinal.
Unfortunately there is no standard notation for ordinals beyond the Feferman-Schütte ordinal ?0. Most systems of notation use symbols such as ?(?), ?(?), ??(?), some of which are modifications of the Veblen functions to produce countable ordinals even for uncountable arguments, and some of which are "collapsing functions".
The smaller Ackermann ordinal is the limit of a system of ordinal notations invented by Ackermann (1951), and is sometimes denoted by or or . Ackermann's system of notation is weaker than the system introduced much earlier by Veblen (1908), which he seems to have been unaware of.