VFR Over the Top

With respect to the operation of aircraft, means the operation of an aircraft over-the-top under VFR when it is not being operated on an IFR flight plan. (FAR definition)

A VFR operation in which an aircraft operates in VFR conditions on top of an undercast. (PHAK definition).

Source: 14 CFR Federal Aviation Regulations §1, Pilot Handbook of Aeronatical Knowledge