The term 'Harmonium' refers to a type of musical instrument. It is related to the ORGAN, melodeon, ACCORDION, and reedorgan. The ORGAN is a keyboard instrument that produces sound by air being pushed through pipes. The melodeon is a type of ORGAN that produces sound by air being pushed through metal reeds. The ACCORDION is a type of instrument that produces sound by air being pushed through metal reeds and buttons. The reedorgan is a type of ORGAN that produces sound by air being pushed through metal reeds. All of these instruments are related to the harmonium, which is a type of keyboard instrument that produces sound by air being pushed through metal reeds.