A <lip> is a movable fold at the entrance to the <mouth>, with <skin> externally and oral mucosa internally. The <lips> contain the <orbicularis oris muscle>, whose contraction helps close and shape the oral opening. Their sensory supply helps guide contact with food and objects.<Anatomy and Physiology 2e by J. Gordon Betts and others>[Section 23.3, The Mouth: lips, mucosa and orbicularis oris]{ref}

The oral seal has several functions. During eating it helps retain food; during <speech>, closure and release can interrupt <airflow>, while rounding changes the geometry of the <vocal tract>. A bilabial stop such as /p/ uses contact between both <lips>, whereas a rounded <vowel> uses an open but reshaped outlet. The visible fold and the <muscle> controlling it are related structures, rather than identical anatomical objects.


 Lip (source code)