labial vestibule

A narrow, natural space between the exteriors of the teeth and the interiors of the lips and cheeks. The vestibule is bordered along its cranial and caudal aspects by the vestibular fold from where the attached gingiva merges into the mobile mucosa.
Definition from Glossary
the portion of the oral cavity that is bounded on one side by the teeth, gingiva, and alveolar ridge (in the edentulous mouth, the residual ridge) and on the other by the lips anterior to the buccal frenula
lā′bē-al ve˘s′ta-byōōl