An anti-grav is 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.
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. (TOS: "Obsession")
By the 24th century, these devices were upgraded into the larger anti-grav sled. Wheelchairs using anti-grav units were also available. (DS9: "Melora")