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 <var>O</var> is usually denoted by
![](http://info.babylon.com/onlinebox.cgi?rt=GetFile&uri=!!ARV6FUJ2JP&type=0&index=4124)
. One of the most widely used, but controversial, examples of an observable physical effect that results from the vacuum expectation value of an operator is the
Casimir effect.