I came across a question in Quora asking the difference between altitude and flight level. One of the answers said that anything above the transition altitude is referred to as a flight level. So suppose an aircraft is flying at 35,000ft, it is flight level 350. Is this correct? Can we not refer to altitude as flight level below 18,000ft (the transition altitude)?
Yes, altitude is below transition level, flight level is above transition altitude.
Altitude is referencing to actual (local) air pressure, while flight level is against standard pressure 2992 / 1013.
The transition altitude is dependent on the location and are definded so both will not mix up.
When climbing through transition altitude, pilots set their QNH to 2992 - and so no longer need to set it to changes due to local weather.
When decending through transition level they set it back to the local QNH - as they are close enough to ground the real altitude (above ground...) is getting important.