cruise-climb

To slowly but continuously climb during cruise flight as the aircraft's weight decreases due to fuel burnoff (done because flight at higher altitudes is more efficient but requires a lighter aircraft).

For a transatlantic crossing, the Concorde would be assigned a ~15,000-foot block of altitude; it started out at flight level 450 and gradually cruise-climbed up to between FL570 and FL600 over the mid-Atlantic before beginning to descend.

An instance of cruise-climbing.

A cruise-climb is more efficient than the standard step climb, but is rarely used since it's much harder for ATC to manage.