Plasma couplings were devices that kept the plasma flow from breaking through the plasma conduits aboard starships.
In 2375, the USS Voyager crew used plasma couplings to transfer antimatter to the Varro generational ship, whose warp drive had been damaged by silicon-based parasites. (VOY: "The Disease")
In 2376, Neelix traded a few of Voyager's plasma couplings to Abaddon for a "cultural artifact", which turned out to be a valuable beryllium crystal. (VOY: "Alice")