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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.^[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.

The orbicularis oris muscle is a complex of skeletal muscle fibres around the mouth that closes and shapes the lips. Its contraction can press the lips together or project them forward. Compare sealing the mouth while holding water with rounding the lips to whistle: both use the same muscular region to change the outlet's shape. The visible lip includes other tissues besides the muscle that moves it.^[ref]