There are at least two instruments identified as melodeons. Both are hand-operated wind instruments, with bellows, free reeds and keyboards.
The first type of melodeon includes small reed organs manufactured in the United States in the early 1800s. Known also as lap or elbow organs, these generally have a single keyboard, one or two sets of free reeds (that is, tuned reeds, one for each note), and bellows operated by your elbow or hand.
A nineteenth-century rectangular button organ, with bellows and sometimes keys, is also called a melodeon. Button accordions have been mass-produced and sold by German firms extensively from the mid- to late-1800s continuing through the 1900s. From Poland to Macedonia, these instruments remain very popular. In North America, the versatile button accordions are featured in Tejano conjuntos (by such renowned performers as Santiago and Flaco Jimenez), in Chicken Scratch bands from the American southwest, in EuroAmerican polka bands, and in accordion and fiddle groups among the Northern Cree, to name only a few (and possibly unexpected) contexts.
More information: Accordion and Keyboard Mechanisms.