MOSFET Labeling 2/1 4/1 16/1
It's possibly channel width to channel length: -



Here's apossibly a simpler view: -



And this might be a fairly informative image: -



I meant to add that I searched for "W/L ratio MOSFET" and also this document might prove useful reading because it covers the W/L ratio in both the ohmic region and the saturation region.