Definition of an accordion: An ill wind that nobody blows good. Answer to Tim: A melodeon is an accordion and is diatonic (different note push-pull) like the button key accordion. It mainly differs in the number and arrangement of the bass (left hand) keys and treble (right hand) voice (reed) switches or couplers. What gives the accordion its sound is two or more reeds playing the same note in different octaves. This produces the "beating" or vibratto sound.