Wiktionary
n. (context science fiction English) Any of various concepts, systems or devices that would oppose or cancel out the force of gravity.
Wikipedia
Anti-gravity is an idea of creating a place or object that is free from the force of gravity. It does not refer to the lack of weight under gravity experienced in free fall or orbit, or to balancing the force of gravity with some other force, such as electromagnetism or aerodynamic lift. Anti-gravity is a recurring concept in science fiction, particularly in the context of spacecraft propulsion. Examples are the gravity blocking substance "Cavorite" in H. G. Wells' The First Men in the Moon and the Spindizzy machines in James Blish's Cities in Flight.
In Newton's law of universal gravitation, gravity was an external force transmitted by unknown means. In the 20th century, Newton's model was replaced by general relativity where gravity is not a force but the result of the geometry of spacetime. Under general relativity, anti-gravity is impossible except under contrived circumstances. Quantum physicists have postulated the existence of gravitons, a set of massless elementary particles that transmit the force, and the possibility of creating or destroying these is unclear.
"Anti-gravity" is often used colloquially to refer to devices that look as if they reverse gravity even though they operate through other means, such as lifters, which fly in the air by moving air with electromagnetic fields.
Usage examples of "anti-gravity".
Using the anti-gravity belt to increase his mobility, Conway began hopping over and around the brontosaur, with Arretapec, who was in rapport with the patient, reporting constantly on the effects of the various stimuli.
Its speed, Blord remembered, had been about thirty thousand miles an hour before he had turned on the superspeed anti-gravities.
These people fought anti-gravity gliden and energy weapons, he thought, and this clown thinks he's going to caw them with a few helicopters and a demeaning name.
A little green man put a slice and three anti-gravity disks on the counter.