Switching threshold (VM): definition

Beginner

The input level where a gate is right on the edge between reading a 0 and a 1.

Novice

The input voltage at which the output equals the input on the transfer curve. Below it the gate treats the input as mostly a 0; above it, mostly a 1. Ideally it sits at half the supply voltage.

Expert

$V_{\mathrm{M}}$ , where $V_{\mathrm{in}} = V_{\mathrm{out}}$ and both devices are saturated. Set by the PMOS/NMOS strength ratio and the thresholds; it moves only slowly (with the square root of the strength ratio in the square-law model) as you resize the devices.