In quantum field theory the vacuum expectation value (also called condensate or simply VEV) of an operator is its average or expectation value in the vacuum. The vacuum expectation value of an operator O is usually denoted by ⟨ ⟨ O ⟩ ⟩ . One of the most widely used examples of an observable physical effect that results from the vacuum expectation value of an operator is the Casimir effect.