Transition altitude (TA)

What is Transition altitude (TA)?

Transition altitude (TA) meaning Altitude in the vicinity of an aerodrome at or below which the vertical position of an aircraft is controlled by reference to altitude (with the aerodrome QNH set on its altimeter). Above transition altitude QNE is set and flight levels used. Also called transition level (TL) at which a descending aircraft changes from FL to QNH.