On-resistance
Beginner
How hard it is for electricity to squeeze through a switch that is on. Lower is better.
Novice
The resistance between source and drain when the transistor is on. It falls as the gate voltage rises and as the transistor is made wider. Designers quote it as a number times the width, such as kΩ·µm.
Expert
Usually an effective resistance averaged over a switching event, chosen so that delay . A common estimate is about . It scales as , which makes of a transistor driving its own gate width-independent.