[dahy-uh-tuhm, -tom]
any of numerous microscopic, unicellular, marine or freshwater algae of the phylum Chrysophyta, having cell walls containing silica.

1835–45; < Neo-Latin Diatoma originally a genus name, feminine noun based on Greek diátomos cut in two. See dia-, -tome

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World English Dictionary
diatom (ˈdaɪətəm, -ˌtɒm)
See also diatomite any microscopic unicellular alga of the phylum Bacillariophyta, occurring in marine or fresh water singly or in colonies, each cell having a cell wall made of two halves and impregnated with silica
[C19: from New Latin Diatoma (genus name), from Greek diatomos cut in two, from diatemnein to cut through, from dia- + temnein to cut]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

1845, coined from Gk. diatomos "cut in two," from diatemnein "to cut through," from dia- "through" + temnein "to cut." So called because they typically appear to have been cut in half. Related: Diatomic.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Science Dictionary
diatom   (dī'ə-tŏm')  Pronunciation Key 
Any of various microscopic protists of the phylum Bacillariophyta that live in both fresh and marine water, have hard bivalve shells (called frustules) composed mostly of silica, and often live in colonies. Most diatoms can perform photosynthesis. They make up a large portion of the marine plankton and are an important food source for many aquatic animals. The skeletal remains of diatoms are the main constituent of diatomite.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
In the right conditions, the pressure of the sediment fill turns the diatom remains into petroleum.
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