Plain English Meaning
It is the little number box on an altimeter where you set the local air pressure. When you turn the knob, the number in that box changes, and the altimeter's altitude reading shifts to match.
Full Definition
The small window on the face of a sensitive altimeter that displays the barometric pressure setting the altimeter is referenced to. Rotating the adjustment knob changes the value shown in the window, which in turn shifts the altitude reading on the main dial.
Context Anchor
Seen on the face of a pressure altimeter during preflight, after receiving an altimeter setting, and during altimeter maintenance or accuracy checks.