First uncountable ordinal

The first uncountable ordinal is a mathematical ordinal that represents the supremum of the set of countable ordinals. It is also the first ordinal number without a fundamental sequence. It is commonly denoted as Ω or ω1.

Usage
Ω is commonly used in the study of uncountable ordinals and as an unsticking point in ordinal collapsing functions (OCFs) such as Madore's Psi. It may be utilized to denote a fixed point of a function in such a case, such that the supremum of the sequence  can be denoted as.

It is of interest that Cantor denoted "Absolute Infinity" with Ω, although the set of all ordinals is itself not an ordinal of course.