Plain English Meaning
A little numbered window on the altimeter where the pilot sets the local air pressure so the altimeter shows the right height.
Full Definition
A small opening on the face of an altimeter that displays the current altimeter setting, expressed in inches of mercury or hectopascals/millibars. The pilot adjusts a knob to dial the local sea-level pressure into this window, which calibrates the altimeter to read correct altitude for the current atmospheric conditions.
Context Anchor
Seen when setting or checking the altimeter, especially before takeoff, during descent, or when studying how affects .