Definition
The procedure of adjusting an aircraft's altimeter to the current local altimeter setting (the station pressure reduced to sea level, expressed in inches of mercury or hectopascals) so that the instrument displays the correct altitude above mean sea level. Pilots are required to set the altimeter to the current reported setting of a station along the route and within 100 nautical miles of the aircraft, or, if no such station is available, to the current reported setting of an appropriate available station. When flying at or above 18,000 feet MSL, the altimeter is set to the standard pressure of 29.92 inches of mercury.
Plain English
Turning the small knob on the altimeter to dial in the current local pressure number so the instrument reads the right height above sea level. Below 18,000 feet you use the local number from a nearby airport or weather station; at and above 18,000 feet, everyone uses the same standard number (29.92).
Context Anchor
You do this before takeoff, when receiving weather information, and whenever air traffic control gives a new altimeter setting in flight.
Derivation
Altimeter comes from altitude, meaning height, and meter, meaning a measuring device. Setting means adjusting something to a chosen value. Together, the phrase means adjusting the height-measuring instrument so it uses the correct reference.
Why Pilots Care
Correct altimeter setting prevents altitude errors that can lead to controlled flight into terrain or airspace violations.
Analogy
It is like setting a clock before you use it. The clock may work perfectly, but if it starts from the wrong reference, every time it shows will be wrong.
Grounding Statement
An altimeter senses air pressure, and setting it tells the instrument what local pressure to treat as the reference for showing altitude.
Intuition Check
Setting the altimeter does not move the airplane or change its real height. It only changes the pressure reference the instrument uses to display altitude.
Example Sentence 1
Before taxi, the pilot listened to ATIS and set the altimeter to 30.05, then cross-checked that the indicated altitude matched the published field elevation.
Example Sentence 2
While en route the pilot updates the altimeter setting when ATC issues a new value to keep altitude readings correct.