Definition
The local barometric pressure value, corrected to sea level, that a pilot dials into the altimeter's Kollsman window so the instrument displays altitude above mean sea level accurately for the surrounding area. In the United States it is given in inches of mercury (for example, 29.92 inHg); in most other countries it is given in hectopascals or millibars.
Plain English
The local pressure number you put into your altimeter so it shows the right height above sea level.
Context Anchor
Seen in aviation weather information, airport broadcasts, and abbreviated notices where space is limited.
Derivation
Altimeter combines Latin altus (high) and Greek metron (measure); setting refers to the calibration value. This origin clarifies that the term describes an adjustment to a height-measuring instrument rather than a general pressure report.
Why Pilots Care
An incorrect altimeter setting produces altitude errors that can result in controlled flight into terrain or airspace violations.
Intuition Check
Do not read ALSTG as the altitude itself. It is the pressure setting used to make the altimeter display altitude correctly.
Example Sentence 1
Tower called the altimeter setting as two-niner-niner-two, so the pilot rotated the Kollsman knob until 29.92 appeared in the window.
Example Sentence 2
En route the controller issued a new ALSTG; the pilot adjusted the altimeter and noted a 200-foot change in indicated altitude.