Refer to figure.
ICAO Doc 8168
Transition level. The lowest flight level available for use above the transition altitude.

Transition Altitude (TA)

The TA is usually specified for each aerodrome by the state in which it is located and shall always be published in charts, broadcast by ATC as well as published in AIPs. The height above the aerodrome of the TA shall be as low as possible but not normally less than 3000 ft. The transition altitude is rounded up to the next full 300 m (1000 ft).

In flight when an aircraft is at or below the transition altitude (TA), its vertical position shall be expressed in terms of altitude. At the Transition Altitude (TA), when climbing, change altimeter from local QNH to standard altimeter setting 1013.25 hPa.

Transition Level (TL)

The transition level shall be passed to the aircraft crew in due time prior to approach and landing. TL shall be communicated by the ATIS (Automatic Terminal Information Service) and it shall be confirmed back by the aircraft crew. At the transition level, the altimeter shall already been set to standard altimeter setting of 1013.5 hPa and this is the first available flight level (i.e. the lowest usable flight level) above the transition altitude. Once the en-route flight is at or above the transition level (TL), the vertical position of the aircraft shall be expressed in terms of flight levels.

At the Transition Level (TL), when descending, change altimeter from standard setting 1013.25 hPa used for flight levels to local QNH.

When flyin above transition level the altitudes are refered to as Flight Levels (FL). FL180 is 18 000 ft with altimeter setting as 1013.25 hPa.

NOTES:

1. QFE - The pressure set on the subscale of the altimeter so that the instrument indicates its height above the reference elevation being used.
2. QNH - The pressure set on the subscale of the altimeter so that the instrument indicates its height above sea level.