Also found in: Acronyms.
A phrase, when used in conjunction with altitude assignments, that means that ATC (air traffic control) has offered the pilot the option of starting to climb or descend whenever he or she wishes and conducting the climb or descent at a rate he or she desires. The pilot may level out at any intermediate altitude. However, once the pilot has vacated an altitude, he or she may not return to that altitude without clearance from ATC. The pilot must inform ATC when vacating an altitude.