Labia are the folds of skin on the outside of a woman's sex organs — the parts you would see, not the passage inside. The word is just the Latin for “lips”, and that is the picture: folds that meet. There are two pairs, though people often say labia for the whole set. The thicker outer pair is the labia majora; the thinner inner pair is the labia minora. Though actually, a lot of everyday talk treats “labia” as if it meant only the outer pair, and some people mix the word up with vagina, which is the inside. In class and in clinic it is a calm, medical word, not a rude one, but it names a private part of the body, so the tone stays matter-of-fact. One single fold is a labium; labia is the plural, and that plural is the form you will almost always meet.