Plain English Meaning
A runway with small grooves cut across it so that rainwater can drain away under the tires instead of building up. This helps the tires keep a firm grip on the pavement when the runway is wet.
Full Definition
A runway whose surface has been cut with shallow, evenly spaced transverse grooves to provide channels for water to escape from beneath aircraft tires, improving tire-to-pavement contact and reducing the risk of hydroplaning during landing or takeoff on a wet surface.
Context Anchor
You will see this term in wet-runway, braking, landing distance, and hydroplaning discussions.
Derivation
Groove comes from an old word meaning a channel or furrow. That helps here because the runway grooves act like many small channels that give water a path off the tire contact area.