Definition
The altitude in the vicinity of an airport at or below which the vertical position of an aircraft is controlled by reference to true altitudes, with the altimeter set to the local altimeter setting (QNH). Above this height, vertical position is expressed as a flight level using the standard altimeter setting of 29.92 inches of mercury (1013.2 hPa).
Plain English
It's the height where pilots switch how they read their altimeter. Below it, they use the local pressure setting and read actual altitude. Above it, they use a standard pressure setting and read flight levels instead.
Context Anchor
Seen in altimeter-setting and arrival or departure procedures, especially in international operations where local height references may be used near an airport.
Derivation
From Latin transire, 'to go across' — the height at which the pilot goes across from one altimeter reference system to another.
Why Pilots Care
Using the wrong pressure reference at this point can create altitude errors that affect terrain clearance and vertical separation from other aircraft.
Intuition Check
Transition height does not mean just any height where something changes. In this context, it means a specified local height where the way of stating vertical position changes.
Example Sentence 1
Descending into the terminal area, the crew reset the altimeter to the local setting as they passed through the transition height.
Example Sentence 2
During descent the crew waited for ATC to issue the transition height so they could set the local QNH before continuing on altitudes.