Definition 1 of 2
Plain English Meaning
The speed that lets the airplane travel the farthest forward for each foot of height it loses while gliding. Fly faster or slower than this and the airplane will come down sooner.
Full Definition
The airspeed that produces the greatest forward distance per unit of altitude lost when an aircraft is gliding without engine power. It corresponds to the angle of attack at which the lift-to-drag ratio is highest, giving the flattest possible glide path through the air.
Context Anchor
Used in engine-out training, emergency procedures, and the aircraft checklist or handbook.
Derivation
Glide comes from an old word meaning to move smoothly or slide. In , it points to the airplane moving forward and downward without engine power. Best means the most favorable speed for distance, not simply the most comfortable or slowest speed.