Definitions

Transition Altitude. The altitude at or below which the vertical position of an aircraft is controlled by reference to altitudes.

Transition Level. The lowest flight level available for use above the transition altitude.

Transition Layer. The airspace between the transition altitude and the transition level.

(ICAO, e.g. Doc 4444: PANS-ATM & Doc 8168: PANS-OPS).

North America

In the US and Canada, the transition altitude is fixed at 18000 feet and the airspace above is known as the Standard Pressure Region

Under conditions of QNH at or above 1013 mb, FL180 becomes the lowest useable FL.  If the pressure is lower, the lowest useable FL becomes FL190 or even FL200.  This restriction ensures that a minimum of 1000' vertical separation is maintained between the aircraft at 17000' on QNH and the one at the lowest useable level on Standard Pressure Setting.