Transition Level is defined by ICAO in PANS-ATM as:
Transition level. The lowest flight level available for use above the transition altitude.
Unfortunately "available for use" is subject to interpretation. Different states use different conventions. In the UK, if you're a VFR flight with a QNh of 1012, FL31 is "available for use". If you're an IFR flight, different levels are the lowest "available for use" depending on direction, though most (including MATS Pt 1) would interpret transition level as the lowest multiple of 5 i.e. FL35 in those circumstances. Other states make it the lowest multiple of 10 above the transition altitude, because only those levels are used for IFR, and still others make it lowest multiple of 10 at least 1000 ft above the transition altitude. Again according to ICAO, the transition level must be above the minimum IFR fight altitude for a control area.
As such, transition level doesn't have much use to pilots. The transition altitude is the phrase used in Rule 34, and neither is used in level selection rules in ICAO Annex 2.