I would assume the definition of TA and TL are or should be the same everywhere. However I know that the TA varies in every country, and even varies within a country as well.
In Norway where I fly we have now set TA to 7000'. Before it was different for every airport you flew into. Some had 4000' some had 6000' and some had 7000' all depending on surrounding terrain. Now however every airport has 7000'. This altitiude is measured above the local QNH at that airport. Depending on the QNH and the difference to QNE the TL is set according to a table for every airport at any given time in order to ensure at least 1000' separation between the TA and the TL. On a day with very low QNH the TL would sometimes be as high as FL90 or even higher. On a day with QNH = 1013,25 HPa The TL would be FL80 and therefore have the 1000' separation between the two. As QNH drops the TL needs to be raised in order to maintain at least 1000' separation.
Someone stated above that an aircraft descending to land at an airport sets his altimeter setting to local QNH at TA is wrong. This is done at TL so that when he descends below TL he is flying on the same altimeter setting as everyone else flying below TA. Since the Transition Layer, the airspace between TA and TL is a greyzone in this relation, no aircraft is given clearance to fly in this layer. This is a transition layer and departing and arriving aircrafts are flying on different alitimeter settings. QNH for arriving and QNE for departing.