As stated before, altitudes above "transition level" are known simply as "flight levels".
All you do is drop the last two zeros.
Therefore, "30000 feet" becomes "flight level 300", "25000 feet" becomes "flight level 250" and so on.
Is that what you were after?
Transition level is where the pilots set their altimeters to the "standard altimeter setting" which is 1013.2 millibars or 29.92 inches of Mercury (depending on which scale is used in any given country) when climbing and to the local altimeter setting (Qnh) when descending.
Transition level differs around the world.
In the US, it's 18000 feet. In the UK it's 6000 feet. And here in NZ it's 11000 feet.
Hope this helps!!