The transition altitude is a published height above sea level at which pilots climbing to their cruising level change their barometric altimeter datum from the regional pressure setting to the common international standard setting of 1013.2hPa. This means all altimeters above that altitude read the same at any given level, despite constant natural meteorological changes in atmospheric pressure, or the passage of the aircraft from a region of high pressure to low pressure or vice versa.
Above the transition altitude, altimeter readings are communicated as flight levels, not as height or altitudes. During descent, pilots change the altimeter datum back to the regional pressure setting when passing through the transition level, the lowest flight level above the transition altitude.
Transitional altitudes are local, regional or national and vary considerably between about 3,000ft and 18,000ft. The USA and Canada have a common one of 18,000ft.
Source: Flight International
