A variant form of the capital Greek letter Omega commonly used to denote sample spaces in probability theory.
Examples
Sample space in probability theory
P(A \cup B) \leq P(\varOmega) = 1
Domain notation in complex analysis
f: \varOmega \to \mathbb{C} \text{ is analytic}
Asymptotic notation in computer science
T(n) = \varOmega(n \log n)