noun, plural epitheliums, epithelia [ep-uh-thee-lee-uh] . Biology.
any animal tissue that covers a surface, or lines a cavity or the like, and that, in addition, performs any of various secretory, transporting, or regulatory functions.

1740–50; < Neo-Latin < Greek epi- epi- + thēl() teat + Neo-Latin -ium -ium

epithelial, adjective
interepithelial, adjective
nonepithelial, adjective
subepithelial, adjective
unepithelial, adjective
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World English Dictionary
epithelium (ˌɛpɪˈθiːlɪəm)
n , pl -liums, -lia
an animal tissue consisting of one or more layers of closely packed cells covering the external and internal surfaces of the body. The cells vary in structure according to their function, which may be protective, secretory, or absorptive
[C18: New Latin, from epi- + Greek thēlē nipple]

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Word Origin & History

Mod.L., from Gk. epi upon + thele teat, nipple (see fecund). Related: Epithelial.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

epithelium ep·i·the·li·um (ěp'ə-thē'lē-əm)
n. pl. ep·i·the·li·ums or ep·i·the·li·a (-lē-ə)
Membranous tissue composed of one or more layers of cells separated by very little intercellular substance and forming the covering of most internal and external surfaces of the body and its organs.

ep'i·the'li·al adj.
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American Heritage
Science Dictionary
epithelium   (ěp'ə-thē'lē-əm)  Pronunciation Key 
Plural epithelia
The thin, membranous tissue that lines most of the internal and external surfaces of an animal's body. Epithelium is composed of one or more layers of densely packed cells. In vertebrates, it lines the outer layer of the skin (epidermis), the surface of most body cavities, and the lumen of fluid-filled organs, such as the gut or intestine.

epithelial adjective
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Behind the epithelial lining of the foramen the choroid plexuses of the lateral
  ventricles are joined across the middle line.
The tube is lined by entoderm from which the epithelial lining of the
  respiratory tract is developed.
She isolated potentially cancerous cells, known as epithelial cells.
Nerve cells are modified epithelial cells, organs do not originate from these
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