Plain English Meaning
The lowest altitude you are allowed to fly on a high-altitude RNAV airway (a Q-route). Stay at or above this altitude and you'll have safe terrain clearance and reliable navigation along that segment.
Full Definition
The lowest published altitudes along an RNAV Q-route in the high-altitude structure (FL180 through FL450) that guarantee both acceptable navigation signal reception for area navigation and obstacle clearance along that segment. Q-routes are RNAV-only airways available to aircraft equipped with approved area navigation systems (typically or ), and their MEAs apply specifically to those rather than to conventional .