1.NounA small fold or ridge of tissue that supports or restrains the motion of the part to which it is attached, in particular a fold of skin beneath the tongue, or between the lip and the gum.
"Humans have three oral frenulums: beneath the tongue, between the upper lip and gum, and between the lower lip and gum."
2.NounSuch a ridge of tissue connecting the foreskin to the glans on the underside of the penis; the penile frenulum.
3.NounA bristle or row of bristles on the edge of the hindwings of some lepidoptera which unites the wings by interlocking with a catch on the posterior part of the forewings.