An Etymological Dictionary of Astronomy and Astrophysics

1. The hole in which a key of a lock is inserted.
   

2. → Keyhole Nebula.
   

3. A small, about 600 m wide, region of space close to the Earth where the Earth’s gravity would perturb the trajectory of a passing → Near-Earth Object. The object will receive a gravitational push that will bring it back for a collision in the future. Also called resonance keyhole.

See also: → key; → hole.