from The American Heritage® Dictionary of the English Language, 4th Edition

  • n. A thin, tough, membranous sac that encloses the embryo or fetus of a mammal, bird, or reptile. It is filled with a serous fluid in which the embryo is suspended.

from Wiktionary, Creative Commons Attribution/Share-Alike License

  • n. the innermost membrane of the fetal membranes of reptiles, birds, and mammals; the sac in which the embryo is suspended.

from the GNU version of the Collaborative International Dictionary of English

  • n. A thin membrane surrounding the embryos of mammals, birds, and reptiles.

from The Century Dictionary and Cyclopedia

  • n. In anatomy and vertebrate zoöl., one of the fetal appendages; the innermost one of the membranes which envelop the embryo of the higher vertebrates, as mammals, birds, and reptiles; the lining membrane of a shut sac, familiarly called the “bag of waters,” in which the fetus is contained.
  • n. In entomology, a membrane which surrounds the larva of many insects, as the millepeds (Iulidæ), for some time after they are batched from the egg. It is regarded by some as the analogue of the amnion of a vertebrate.
  • n. A reflected portion of a membrane, in ascidians, which lines the inner wall of the ovisac, and forms a kind of amniotic investment of the embryo.
  • n. In botany, a name formerly given to the fluid contents of the embryo sac.
  • n. Sometimes erroneously written amnios.
  • n. In echinoderms, the sac in the pluteus larva inclosing the developing echinus.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

  • n. thin innermost membranous sac enclosing the developing embryo of higher vertebrates (reptiles, birds and mammals)


Greek amniōn.
(American Heritage® Dictionary of the English Language, Fourth Edition)
From Latin for "membrane around a fetus", from Greek "bowl in which the blood of victims was caught", from ame, (bucket) (Wiktionary)



Log in or sign up to get involved in the conversation. It's quick and easy.

  • I found this word in my reading today, from the memoir A Step from Death, by Larry Woiwode a great word in and of itself:

    " . . . I feel my mind is treading water, so deep in moribund amnion I'll drown."

    September 5, 2008