Gate length (L, L_g)
Beginner
How long the switch’s control strip is, measured in the direction current flows. Shorter means a smaller, faster switch.
Novice
The distance from source to drain under the gate, along the direction current flows. Shorter gates switch faster and fit more transistors in the same space, but the gate loses control when they get too short.
Expert
Physical gate length, usually a little longer than the effective channel length set by the source/drain junctions. It stopped tracking node names in the mid-1990s and has scaled more slowly than gate pitch since; FinFET-era values are around 20 nm (ASAP7 assumes 21 nm).
Explained in Shrinking (Transistors).
See also: Short-channel effects, Contacted poly pitch (CPP, gate pitch).