Nymph is an adult entertainment magazine containing scantily-clad, yet tasteful, photos of women. Reilly possesses one, and when he is killed this copy passes down to Hot Dog (Scar).
Notes
The magazine can be seen among the things Starbuck puts in a box after Reilly is killed in combat. In a scene cut from the episode, the pilots auction off Reilly's possessions and Hot Dog wins the bid for the skin magazine, thus explaining why he reads it later in the rec room.