Medical Definition of lip
1: either of the two fleshy folds which surround the opening of the mouth in humans and many other vertebrates and in humans are organs of speech essential to certain articulations; also : the pinkish or reddish margin of a human lip composed of nonglandular mucous membrane and usually exposed when the mouth takes on its natural set
2: an edge of a wound
3: either of a pair of fleshy folds surrounding an orifice
4: an anatomical part or structure (as a labium) resembling a lip
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