The threshold voltage, commonly abbreviated as V_th or V_{GS (th)}, of a field-effect transistor (FET) is the minimum gate-to-source voltage differential that is needed to create a conducting path between the source and drain terminals.

For an nMOS device at gate-to-source voltages above the threshold voltage ((V_GS > V_th) but still below saturation (less than "fully on", (V_GS − V_th) > V_DS), the transistor is in its 'linear region', also known as ohmic mode, where it behaves like a voltage-controlled variable resistor.