mi·cro·or·gan·ism (m??kr?-ôr?g?-n?z??m)n. An organism or infectious agent of microscopic or submicroscopic size, especially a bacterium or protozoan.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.microorganism (?ma?kr????????n?z?m) n (Microbiology) any organism, such as a bacterium, protozoan, or virus, of microscopic sizeCollins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014mi?cro?or?gan?ism (?ma? kro???r g??n?z ?m) n. any organism too small to be viewed by the unaided eye, as bacteria or some fungi and algae. [1875?80] mi`cro?or?gan?ic (-?r?gæn ?k) adj. Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.mi·cro·or·gan·ism (m??kr?-ôr?g?-n?z??m) An organism that can be seen only with the aid of a microscope. Microorganisms include bacteria, protozoans, algae, and fungi. See Note at germ.The American Heritage® Student Science Dictionary, Second Edition. Copyright © 2014 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.microorganismany one of a large variety of microscopic or ultramicroscopic organisms, as bacteria, viruses, etc.See also: Organisms-Ologies & -Isms. Copyright 2008 The Gale Group, Inc. All rights reserved.