Subthreshold swing (slope)
Beginner
How sharply a transistor shuts off as its gate voltage drops below the tipping point. Sharper means less leaking.
Novice
The gate-voltage change needed to cut the off-state current by a factor of ten. Smaller is better: the switch turns off more completely for the same voltage.
Expert
, with a thermal limit of about 60 mV/decade at room temperature; real devices sit above it, and about 100 mV/decade is a common textbook planning value. It sets how many decades of leakage you buy per volt of .
Explained in The switch (Transistors).
See also: Threshold voltage (Vt), Leakage (static) power.