Threshold Voltage (VGS(th))
The gate-source voltage at which a MOSFET starts to conduct; typically 2-4 V in standard power types and around 1-2 V in logic level types. This value is the point where conduction begins; full conduction usually requires a gate voltage of 10-12 V.
A common mistake is to take the threshold voltage for the voltage that fully turns the MOSFET on. At the threshold the channel has only opened enough to pass a few milliamperes. In datasheets the channel resistance is usually given for a gate voltage of 10 V; if you drive a standard power MOSFET with 5 V the resistance comes out much higher and the device overheats.
Logic level MOSFETs are made with a lower threshold to solve this problem and reach their declared channel resistance with a gate voltage of 4.5-5 V. This type is preferred in designs driven directly from a microcontroller output.
The second property of the threshold voltage is that it falls with temperature; typically it decreases by a few hundred millivolts per 100 °C rise. That is why a MOSFET in a hot power stage turns on unintentionally from noise more easily. The pull-down resistor placed between gate and source and low-impedance drive are there to prevent this unintended turn-on.
The context in which this term is used: DC Motor Driver Selection Guide