Overdrive voltage, usually abbreviated as VOV, is typically referred to in the context of MOSFET transistors.
"[1] Overdrive voltage can be found using the simple equation: VOV = VGS − VTH.
The table below shows how to use overdrive voltage to understand what region of operation the MOSFET is in: A more physics-related explanation follows: In an NMOS transistor, the channel region under zero bias has an abundance of holes (i.e., it is p-type silicon).
By applying a negative gate bias (VGS < 0) we attract more holes, and this is called accumulation.
As we increase this voltage, VGS, beyond VTH, we are said to be then overdriving the gate by creating a stronger channel, hence the overdrive (often called Vov, Vod, or Von) is defined as (VGS − VTH).