A portable antigravity unit clamped to heavy or dangerous objects so that one or two people can carry them, used by Starfleet crews from Kirk's time onwards.
An antigrav is a hand-carried device that cancels the weight of whatever it is fixed to. Starfleet crews use them to move cargo, machinery and hazardous materials that would otherwise need lifting gear, and larger versions take the form of floating pallets and sleds.
In Obsession, Kirk and Ensign Garrovick use an antigrav unit to carry a container of antimatter down to Tycho IV as a bomb to destroy the vampire cloud creature.
A faulty one opens the mystery in Hollow Pursuits. An antigrav unit fails in a cargo bay of the Enterprise-D while moving medical containers, one of them breaks, and it becomes the first of a chain of malfunctions that Reginald Barclay eventually traces to contamination by invidium.