Virtual state

A short-lived, unobservable quantum state

In quantum physics, a virtual state is a very short-lived, unobservable quantum state. In many quantum processes a virtual state is an intermediate state, sometimes described as "imaginary" in a multi-step process that mediates otherwise forbidden transitions. Since virtual states are not eigenfunctions of any operator, normal parameters such as occupation, energy and lifetime need to be qualified.

From Wikipedia, under CC BY-SA. More on occurri.