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