Sidewall spacer
Beginner
A thin strip of insulator left on each side of a gate, used to keep the next implant a small distance away from it.
Novice
A conformal insulating film deposited over the gates and etched straight down, which leaves material only on the gate’s vertical sides. It sets how far the heavy source/drain doping sits from the channel.
Expert
Formed by conformal deposition and anisotropic etch-back, so its width is set by film thickness, not lithography. The same trick at larger scale is spacer patterning (SADP/SAQP) for fins, gates and tight metal pitches.
Explained in Making them (Transistors).
See also: Self-aligned gate, Multiple patterning.