The term Melodeon has several meanings and refers to different types of musical instruments across the world.   In the UK it is the name generally used for referring to the Diatonic Button Accordion.  

I play a two row 'forth apart’ system in the keys of D and G (and others!).   The melody is played on the two rows with the right hand and the bass and chord accompaniment played on the left hand. 

The melodeon is great for playing for social dance such as Morris Dancing and Ceilidhs,   playing along with other folk instruments in a music session or for accompanying vocals.