Plain English Meaning

It's the small knob on the altimeter you turn to dial in the current pressure setting given by ATC or ATIS, so the altimeter shows the right altitude.

Full Definition

The knob on a sensitive altimeter used to set the local barometric pressure (in inches of mercury or hectopascals) into the altimeter's Kollsman window, so the instrument reads altitude relative to the correct pressure reference.

Context Anchor

Seen on the face of a sensitive altimeter, especially when setting the altimeter , during , or when given a new by .