[Transition Altitude - The highest altitude at which an aircraft in normal operation should use an altimeter pressure setting indicating height above mean sea level.
Transition Level - The lowest flight level available for use above the transition altitude.
Which probably isn't very enlightening if the reader doesn't know that at some certain altitude, all aircraft set their altimeters to the same setting, regardless of the local barometric pressure. That's why it's necessary to define a "transition altitude". Below that altitude, you could have a substantially different altimeter reading than you would above it, because the altimeter settings could be substantially different. The result could be less than the required altitude seperation between two aircraft.
More info than anyone here would care to type can be found in this Wikipedia article:
http://en.wikipedia.org/wiki/Flight_level