Vaginal membrane: A thin membrane which completely or partially occludes the vaginal opening. This fold of mucous membrane is usually present at birth at the orifice of the vagina. It is also called the hymen, a Greek word meaning "skin" or "membrane." The ancient Greeks applied the word "hymen" 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.