Plain English Meaning
It is the little window on the altimeter that shows the pressure number the instrument is set to. The pilot turns a knob to put the right number in this window so the altimeter shows the correct height.
Full Definition
A small opening on the face of a sensitive altimeter that displays the current barometric pressure setting (in inches of mercury or hectopascals/millibars) to which the instrument is calibrated. The pilot adjusts this setting using the knob on the altimeter so that the instrument reads correct altitude based on the local atmospheric pressure.
Context Anchor
Seen on the face of a pressure altimeter during , , during setup, and anytime a new is received from or .