ep·i·the·li·um 
(ĕp′ə-thēlē-əm)
(ĕp′ə-thēlē-əm)n. pl. ep·i·the·li·a (-lē-ə) or ep·i·the·li·ums
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.
[New Latin epithēlium : EPI- + Greek thēlē, nipple; see dhē(i)- in the Appendix of Indo-European roots.]
ep′i·theli·al adj.