Plain English Meaning
The little number window on the altimeter where you dial in the current air pressure setting given by ATC or the weather report. Setting it correctly makes the altimeter show the right altitude.
Full Definition
A small adjustable scale on an altimeter, visible through a window in the instrument face, that allows the pilot to set the current local atmospheric pressure (in inches of mercury or millibars/hectopascals) so the altimeter reads accurate altitude relative to that pressure setting.