Plain English Meaning
The lowest altitude you are allowed to fly on a particular RNAV (GPS-based) route segment. At or above this altitude, you are guaranteed to clear terrain and obstacles and to have reliable navigation along that segment.
Full Definition
The lowest published altitude on an RNAV (area navigation) route segment that guarantees both adequate obstacle clearance and acceptable navigation signal reception or RNAV system performance for the entire segment between the two fixes that bound it. On high-altitude charts, an RNAV Route MEA is identified by the route designator with the prefix Q (e.g., Q-145) and is shown alongside other charted for the segment.