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microorganism

[mahy-kroh-awr-guh-niz-uh m]
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noun
  1. any organism too small to be viewed by the unaided eye, as bacteria, protozoa, and some fungi and algae.
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Origin of microorganism

First recorded in 1875–80; micro- + organism
Related formsmi·cro·or·gan·ic [mahy-kroh-awr-gan-ik] /ˌmaɪ kroʊ ɔrˈgæn ɪk/, mi·cro·or·gan·is·mal, adjective
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

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British Dictionary definitions for microorganism

microorganism

noun
  1. any organism, such as a bacterium, protozoan, or virus, of microscopic size
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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

microorganism in Medicine

microorganism

(mī′krō-ôrgə-nĭz′əm)
n.
  1. An organism of microscopic or submicroscopic size, especially a bacterium or protozoan.
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The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

microorganism in Science

microorganism

[mī′krō-ôrgə-nĭz′əm]
  1. 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.
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The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.