The set, denoted by ∅ with no elements in it. Consequently, its cardinality is zero.
Statistics
Seesample space.
Computer
A set with no elements. It is usually denoted by φ.
Philosophy
The set that has no members (there is only one, since sets are different only if they have different members). The existence of this set as a mathematical object can be puzzling if one is (incorrectly) inclined to visualize a set as a kind of heap.