Plain English Meaning
How well the aircraft turns its motion through the air into lift without paying too much in drag. Higher efficiency means more lift for less drag.
Full Definition
The ratio of useful aerodynamic output (lift) to the cost of producing it (drag), expressed as the lift-to-drag ratio (L/D). An aircraft is more aerodynamically efficient when it produces more lift for each unit of drag, allowing it to fly farther, glide better, and use less power for a given performance.
Context Anchor
Seen in aircraft performance, stability, trim, and center-of-gravity discussions, especially when describing how aircraft loading or can increase drag.