Before revision: The overdrive voltage is defined as the voltage between transistor gate and source (VGS) in excess of the threshold voltage (VTH) where Vt is defined as the minimum voltage required between gate and source to turn the transistor on (allow it to conduct electricity).
After revision: The overdrive voltage is defined as the voltage between transistor gate and source (VGS) in excess of the threshold voltage (VTH) where VTH is defined as the minimum voltage required between gate and source to turn the transistor on (allow it to conduct electricity).