The Collaborative International Dictionary of English v.0.48:

Amnion \Am"ni*on\, n. [Gr. ? the membrane round the fetus, dim.
   of ? lamb.] (Anat.)
   A thin membrane surrounding the embryos of mammals, birds,
   and reptiles.
   [1913 Webster]

WordNet (r) 3.0 (2006):

amnion
    n 1: thin innermost membranous sac enclosing the developing
         embryo of higher vertebrates (reptiles, birds and mammals)
         [syn: amnion, amniotic sac, amnios]

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "amnion":
   allantoic membrane, amniotic sac, arachnoid membrane,
   basement membrane, conjuctiva, eardrum, hymen, maidenhead,
   membrana serosa, membrana tympana, membrane, meninges, meninx,
   neurilemma, pellicle, pericardium, perineurium, pleura, serosa,
   tympanic membrane, tympanum, velum