pla·cen·ta (pl?-s?n?t?)n. pl. pla·cen·tas or pla·cen·tae (-t?) 1. a. A membranous vascular organ that develops in female eutherian mammals during pregnancy, lining the uterine wall and partially enveloping the fetus, to which it is attached by the umbilical cord. Following birth, the placenta is expelled.b. A similar organ in marsupial mammals, consisting of a yolk sac attached to the uterine wall.c. An organ with similar functions in some nonmammalian animals, such as certain sharks and reptiles.2. Botany The part within the ovary of a flowering plant to which the ovules are attached.[New Latin, from Latin, flat cake, alteration of Greek plakoenta, from accusative of plakoeis, flat, from plax, plak-, flat land, surface; see pl?k- in Indo-European roots.]pla·cen?tal adj.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.placenta (pl??s?nt?) n, pl -tas or -tae (-ti?) 1. (Anatomy) the vascular organ formed in the uterus during pregnancy, consisting of both maternal and embryonic tissues and providing oxygen and nutrients for the fetus and transfer of waste products from the fetal to the maternal blood circulation. See also afterbirth2. (Zoology) the corresponding organ or part in certain mammals3. (Botany) botany a. the part of the ovary of flowering plants to which the ovules are attachedb. the mass of tissue in nonflowering plants that bears the sporangia or spores[C17: via Latin from Greek plakoeis flat cake, from plax flat]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014pla?cen?ta (pl??s?n t?) n., pl. -tas, -tae (-t?). 1. the organ in most mammals, formed in the lining of the uterus by the union of the uterine mucous membrane with the membranes of the fetus, that provides for the nourishment of the fetus and the elimination of its waste products. 2. a. the part of the ovary of flowering plants that bears the ovules. b. (in ferns and related plants) the tissue giving rise to sporangia. [1670?80;