What is "vfr-on-top"

In United States aviation VFR-on-top is defined as air traffic control authorization for an aircraft operating under instrument flight rules (IFR) to do so in visual meteorological conditions (VMC) at any appropriate visual flight rules (VFR) altitude. A pilot receiving this authorization must comply with the VFR visibility, distance from cloud criteria, and the minimum VFR altitudes. The use of this term does not relieve controllers of their responsibility to separate aircraft in Class B and Class C airspace or TRSAs.