• synonyms


[am-nee-uh n]
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noun, plural am·ni·ons, am·ni·a [am-nee-uh] /ˈæm ni ə/.
  1. Anatomy, Zoology. the innermost of the embryonic or fetal membranes of reptiles, birds, and mammals; the sac in which the embryo is suspended.
  2. Zoology. a similar membrane of insects and other invertebrates.
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Origin of amnion

1660–70; < Greek, equivalent to amn(ós) lamb (see yean) + -ion diminutive suffix
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Related Words for amnion

afterbirth, amnion, placenta

Examples from the Web for amnion

Historical Examples of amnion

  • Amnion: the inner of the two membranes enveloping the embryo.

    Explanation of Terms Used in Entomology

    John. B. Smith

  • The embryo when about to hatch throws off the egg-shell and amnion in a few seconds.

    Our Common Insects

    Alpheus Spring Packard

  • Only in the chancel were they protected by sheep's amnion stretched on frames.

    Pabo, The Priest

    Sabine Baring-Gould

  • The amnion contains a fluid known by the name of liquor amnii.

  • Both it and the stalk of the yolk-sack are enveloped by the amnion (am).

British Dictionary definitions for amnion


noun plural -nions or -nia (-nɪə)
  1. the innermost of two membranes enclosing an embryonic reptile, bird, or mammalSee also chorion
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Word Origin for amnion

C17: via New Latin from Greek: a little lamb, from amnos a lamb
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

Word Origin and History for amnion


1660s, Modern Latin, from Greek amnion "membrane around a fetus," said to be originally "bowl in which the blood of victims was caught" [Liddell & Scott], which is variously said to be of unknown origin, from ame "bucket," or a diminutive of amnos "lamb."

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Online Etymology Dictionary, © 2010 Douglas Harper

amnion in Medicine


(ămnē-ən, -ŏn′)
n. pl. am•ni•ons
  1. The thin, membranous sac filled with a serous fluid in which the embryo or fetus is enclosed and suspended in the uterus.amniotic sac
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Related formsam′ni•otic (-ŏtĭk) null adj.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

amnion in Science


  1. A thin, membranous sac filled with a watery fluid (called the amniotic fluid) in which the embryo or fetus of a reptile, bird, or mammal is suspended during prenatal development. Also called amniotic sac
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The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.