A Klingon delicacy made of serpent worms, which Klingons insist is at its best when the worms are still alive and moving on the plate.
Gagh is introduced in the episode A Matter of Honor. Before joining the Klingon ship IKS Pagh as an exchange officer, Riker samples Klingon food in Ten Forward to prepare himself. In the Pagh's mess hall he is then served the real thing, alive, while the crew watch to see whether the human will eat it. He does, and wins some respect.
The dish becomes a regular reference in later series. On Deep Space Nine the Klingon restaurant on the Promenade serves it, and the dish is a favourite of Jadzia Dax.
In Prodigal Daughter her successor Ezri has to deal with a large shipment that Jadzia had ordered before her death, and reels off the different varieties while admitting that she cannot stand any of them.