The term 'Xylophone Cousin' refers to three different instruments that are related to the xylophone. The first is the glockenspiel, which is a percussion instrument consisting of a set of tuned metal bars that are struck with mallets. The second is the MARIMBA, which is a percussion instrument made of wooden bars and resonators that are struck with mallets. Finally, the VIBES is a type of xylophone that has metal bars and is played with mallets. All three instruments are related to the xylophone and are commonly referred to as its cousins.