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.
Plain English
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.
Context Anchor
Seen on the face of a pressure altimeter during preflight, before takeoff, during approach setup, and anytime a new altimeter setting is received from weather or air traffic control.
Derivation
Called the altimeter setting window because it is the window through which the pilot sees and sets the pressure value the altimeter uses as its reference. Sometimes also called the Kollsman window, after Paul Kollsman, who developed the first practical sensitive altimeter in 1928.
Why Pilots Care
An incorrect value here causes the altimeter to show the wrong altitude, which can lead to controlled flight into terrain or airspace violations.
Intuition Check
The window is not a cockpit window or an outside view. Here, window means the small display area on the instrument where the selected pressure setting is shown.
Example Sentence 1
Before taxi, the pilot set 30.05 in the altimeter setting window after receiving the current altimeter setting from ATIS.
Example Sentence 2
After receiving the new weather report, she reset the altimeter setting window to match the updated local pressure.