Hymen: A thin membrane which completely or partially occludes the vaginal opening. The hymen is a fold of mucous membrane usually present at birth at the orifice of the vagina. It is also called the vaginal membrane (or, sometimes, the virginal membrane).
"Hymen" is a Greek word meaning "skin" or "membrane." The ancient Greeks applied the word to all kinds of membranes including, for example, the membrane that surrounds the heart (the pericardium). Starting in the 1500s the word was restricted to refer only to the vaginal membrane.