Refer to figures.
Transition altitude → The altitude at or below which the vertical position of an aircraft is controlled by reference to altitudes using QNH.

Transition level → The lowest flight level available for use above the transition altitude. At or above the transition level 1013.2 hPa (QNE) pressure setting is used and the altimeter indicates flight levels.

Transition layer → The airspace between the transition altitude and the transition level. When climbing through this layer 1013.2 hPa setting is used and the altimeter indicates flight levels, and when descending through this layer QNH is used and the altimeter indicates altitudes.

The transition level is the lowest flight level available for use ABOVE the transition altitude. Therefore, it is always higher than the transition altitude.