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8 Words That Are Older Than You Think

microorganism

[mahy-kroh-awr-guh-niz-uh m] /ˌmaɪ kroʊˈɔr gəˌnɪz əm/
noun
1.
any organism too small to be viewed by the unaided eye, as bacteria, protozoa, and some fungi and algae.
Origin
1875-1880
1875-80; micro- + organism
Related forms
microorganic
[mahy-kroh-awr-gan-ik] /ˌmaɪ kroʊ ɔrˈgæn ɪk/ (Show IPA),
microorganismal, adjective
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Based on the Random House Dictionary, © Random House, Inc. 2014.
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British Dictionary definitions for micro organism

microorganism

/ˌmaɪkrəʊˈɔːɡəˌnɪzəm/
noun
1.
any organism, such as a bacterium, protozoan, or virus, of microscopic size
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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micro organism in Medicine

microorganism mi·cro·or·gan·ism (mī'krō-ôr'gə-nĭz'əm)
n.
An organism of microscopic or submicroscopic size, especially a bacterium or protozoan.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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micro organism in Science
microorganism
  (mī'krō-ôr'gə-nĭz'əm)   
An organism that can be seen only with the aid of a microscope and that typically consists of only a single cell. Microorganisms include bacteria, protozoans, and certain algae and fungi. See Note at germ.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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