Plain English Meaning
How far the airplane can travel forward without engine power before it reaches the ground.
Full Definition
The horizontal distance an airplane can travel through the air after a loss of engine power, while descending from its current altitude to the ground at its best glide speed and configuration.
Context Anchor
You use this idea when deciding whether the airplane can reach a runway or landing area during a power-off glide, especially when wind is changing your path over the ground.
Derivation
Glide comes from an old word meaning to move smoothly. In aviation, it points to the airplane continuing forward while descending, rather than simply dropping straight down.