abstract:In quantum field theory the vacuum expectation value (also called condensate or simply VEV) of an operator is its average, expected value in the vacuum. The vacuum expectation value of an operator O is usually denoted by \langle O\rangle.
This is reflectedin the fact that the vacuumexpectation valueof the field squaredis nonzero in quantum field theory. Therefore there exist the so calledvacuumfluctuations.