har·mo·ni·um (här-m??n?-?m)n. An organlike keyboard instrument that produces tones with free metal reeds actuated by air forced from a bellows.[French, from harmonie, harmony, from Old French armonie, harmonie; see harmony.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.harmonium (h???m??n??m) n (Instruments) a musical keyboard instrument of the reed organ family, in which air from pedal-operated bellows causes the reeds to vibrate[C19: from French, from harmonie harmony]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014har?mo?ni?um (h?r?mo? ni ?m) n. an organlike keyboard instrument with small metal reeds and pedal-operated bellows. [1840?50; Latinization of Greek harmónion, neuter of harmónios harmonious] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.