Definition
The lowest flight level available for use above the transition altitude. When climbing through the transition altitude, the altimeter is reset from the local pressure setting (QNH) to the standard setting of 29.92 inHg (1013.2 hPa), and the aircraft's vertical position is then expressed as a flight level rather than an altitude.
Plain English
The lowest flight level you can use once you've climbed high enough to switch your altimeter to the standard setting. Below it, you fly by altitude in feet; at or above it, you fly by flight level.
Context Anchor
Seen in altimeter-setting discussions, especially in instrument procedures that use local airport pressure references such as QFE.
Derivation
Transition' comes from Latin 'transitio,' meaning a passing across or change. It marks the point where vertical reference changes from local pressure to a standard pressure setting.
Why Pilots Care
Using the correct transition level keeps vertical separation accurate between aircraft and prevents altitude errors near terrain.
Grounding Statement
Picture descending toward an airport: above the transition level everyone is using the same standard pressure reference, and below it you switch to the local reference for that airport.
Intuition Check
“Transition level” does not mean any level where a change happens. In instrument flying, it specifically means the lowest usable standard-pressure flight level above the transition altitude.
Example Sentence 1
Climbing through the transition altitude, the pilot reset the altimeter to 29.92 and reported reaching the transition level shortly after.
Example Sentence 2
In mountainous areas the published transition level is often raised to maintain safe terrain clearance.