Dulcimer is an instrument that Cake plays, similar to how Jake plays his viola. It appeared in the episode "Fionna and Cake." It was used during the song "Oh, Fionna", which is sung by Prince Gumball, which was actually The Ice Queen in disguise.
The dulcimer is a large brown box, possibly made out of wood. Cake uses 2 stick-like objects to play her dulcimer.