An anti-grav was any device capable of nullifying the force of gravity, usually to lift loads of an extremely high mass. Anti-grav devices were in use primarily during the 23rd century. Certain types of automobiles, such as hovercars and aircars were likely equipped with anti-gravs, making it possible for a car or truck to hover or fly.
At various times these devices were used to transport Nomad, to handle an antimatter bomb, and later to carry the Medeusan Ambassador Kollos around the USS Enterprise. (TOS: "The Changeling", "Obsession", "Is There in Truth No Beauty?") The unit Captain Kirk and Ensign Garrovick used to transport the antimatter bomb was marked ANTI-GRAV 57,000 Gkg, suggesting that particular anti-grav unit had the capacity to amplify motive power to allow 57 kilograms to seem like 1 kilogram at 1 g. (TOS: "Obsession")