A collection of all possible elementary events that can occur in an experiment. Represents with Ω or S. An elementary event is denoted as ω ∈ Ω. If Ω is finite, Ω={ω1,ω2,...,ωn}. If countable or infinite, Ω={ω1,ω2,...}. In continuous cases, Ω is an interval or R^n. Event A is a subset of Ω (A⊆Ω). |Ω| is the cardinal or number of elements