A transport inhibitor was a device that blocked transporters attempting to dematerialize objects in a specific area.
In 2375, Lieutenant Commander Data used a transport inhibitor aboard the Federation mission scoutship to prevent himself from being beamed away by an USS Enterprise-E shuttlecraft. Several portable transport inhibitors were later deployed by Enterprise-E personnel on the surface of Ba'ku to prevent the Son'a from forcibly removing the inhabitants of the Ba'ku village. Due to their large size and easily noticeable positions, several were destroyed by Son'a shuttles. (Star Trek: Insurrection)
In the Tenth UK Story Arc, the Eldorians possessed transport inhibitors as early as 2268. These were used in their war against the Norusians, who used long-range transporters to kidnap enemy personnel.