Livingston is the lionfish kept in an aquarium in Jean-Luc Picard's ready room aboard the Enterprise-D throughout The Next Generation, named on screen in Chain of Command.
Livingston can be seen in the tank set into the wall of the captain's ready room from the first season of The Next Generation to the last, a silent presence in countless scenes between Picard and his officers.
The fish is identified by name only in the sixth-season story Chain of Command. When Edward Jellico takes command of the Enterprise he sets about remaking the ship to suit himself, and one of his instructions to William Riker is to have the fish removed from the ready room. The aquarium is back in place once Picard returns.
The fish takes no part in any plot, but it became one of the best-known fixtures of the Enterprise-D sets and a favourite piece of trivia among viewers.