An antigravity interrupter or gravity neutralizer was an advanced technological device developed in the 23rd century.
History and specifics
In 2266, a Federation antigravity interrupter was under development aboard space station Research Satellite-5. The device projected a "block of energy" between the target and the strongest nearby source of gravity, effectively nullifying the gravity around the target. The device was built in a cylindrical room and consisted of at least two very large pieces of equipment. As a demonstration of the technology, Montgomery Scott was levitated. (TOS comic: "A Bomb in Time")
In 2269, a gravity neutralizer was installed inside a cylindrical earthquake-proof structure on the Mad Planet. It was activated by Prince Tchar of Skorr to prevent a landing party from retrieving a stolen religious artifact, the Soul of Alar. James T. Kirk and Spock took advantage of their training in null-gravity combat exercises to capture Tchar and recover the Soul. (TAS episode & Log Five novelization: The Jihad)