frenulum

/ˈfrɛnjələm/
noun
  1. A small fold of tissue that limits the movement of a body part, such as the one under the tongue or inside the upper lip.
    • He accidentally bit his frenulum while eating, and it bled a little.
    • Some people have a short frenulum that can affect speech.
    • The doctor checked the baby's frenulum to see if it was too tight.