Definition
The barometric pressure value, expressed in inches of mercury (or hectopascals/millibars), that a pilot sets in the altimeter's Kollsman window so the instrument reads the correct altitude above mean sea level for the local area. It is the station pressure corrected to sea level using standard atmospheric assumptions.
Plain English
It's the local air pressure number you dial into your altimeter so it shows the right height above sea level. ATC or an automated weather broadcast gives you this number, and you turn a small knob to enter it.
Context Anchor
You will hear or read the altimeter setting in airport weather reports, from controllers, and before takeoff or landing when setting the cockpit altimeter.
Derivation
From 'altimeter' (Latin altus, 'high' + Greek metron, 'measure') and 'setting' (the value you set into the instrument). The word 'setting' is literal -- you are setting a reference value into the device.
Why Pilots Care
An incorrect value causes the altimeter to display the wrong altitude, which can lead to terrain conflicts or airspace violations.
Grounding Statement
Changing the altimeter setting does not move the airplane; it changes how the altimeter converts air pressure into an altitude reading.
Intuition Check
Altimeter setting is not the altitude shown on the altimeter. It is the pressure number entered so the altimeter can show the correct altitude.
Example Sentence 1
Tower advised, 'Altimeter setting two niner niner two,' and the pilot rotated the knob until 29.92 appeared in the Kollsman window.
Example Sentence 2
While flying cross-country, she updated the altimeter setting when the controller reported a new value of 30.05.