From The Art and Popular Culture Encyclopedia
The labia are anatomical structures that are part of the female genitalia; they are the major externally visible portions of the vulva. In humans, there are two pairs of labia: the outer labia, or labia majora are larger and fattier, while the labia minora are folds of skin often concealed within the outer labia. The labia surround and protect the clitoris and the openings of the vagina and urethra.
Labium (plural labia) is a Latin-derived term meaning "Lip". Labium and its derivatives (including labial, labrum) are used to describe any lip-like structure, but in the English language, labium often specifically refers to parts of the vulva.