A class J nebula was a type of nebula.
The USS Voyager entered a class-J nebula in 2376 to collect deuterium, where it inadvertently picked up an electromagnetic lifeform. It later returned the lifeform to a similar nebula. (VOY: "The Haunting of Deck Twelve")
Class J nebulae containing ionized gas could contain a warp core explosion. The Delta Flyer ejected its sabotaged warp core into such a nebula during the Antarian Trans-stellar Rally in 2377, to protect those attending the race. (VOY: "Drive")