A powerful, bright blue alcoholic drink from the Romulan Star Empire, banned in the Federation for much of its history yet regularly found on Starfleet tables.
Romulan ale is first seen in Star Trek II: The Wrath of Khan, when McCoy brings a bottle to Kirk's apartment as a birthday present. Kirk points out that it is illegal; McCoy says he uses it for medicinal purposes.
In Star Trek VI: The Undiscovered Country it is served at the uneasy dinner for Chancellor Gorkon's party aboard the Enterprise-A, and the officers are still feeling its effects when Gorkon is assassinated later that night.
The ban is briefly lifted during the Dominion War, when the Romulans are allies, as mentioned in the Deep Space Nine episode Inter Arma Enim Silent Leges. In Star Trek Nemesis the drink flows at the wedding of Riker and Troi, leaving Worf to mutter that it ought to be illegal.