labum - n.
(pl. labia)
1 (usu. in pl.) Anat. each of the two pairs of skin folds that enclose the vulva.
2 the lower lip in the mouth-parts of an insect or crustacean.
3 a lip, esp. the lower one of a labiate plant's corolla.

labia majora the larger outer pair of labia (in sense 1). labia minora the smaller inner pair of labia (in sense 1). [L, = lip]