The term ‘incompetent lips’ specifically describes lips that fail to close naturally at rest when the mandible is in its resting position. This condition indicates an inability to maintain a proper lip seal without conscious effort, often leading to mouth breathing, drooling, or altered oral posture. It is critical in assessing facial muscle function and planning treatments in orthodontics and maxillofacial surgery. Lips that do not seal at rest compromise oral health by increasing the risk of dryness, caries, and periodontal disease. Hence, the defining clinical feature of incompetent lips is their failure to achieve passive closure at mandibular rest position.

Reference: Contemporary Orthodontics, William R. Proffit, 6th Edition.