frenum vs. frenulum

frenum

[free-nuhm] / ˈfri nəm /

noun

Anatomy, Zoology.

a fold of membrane that checks or restrains the motion of a part, such as the fold on the underside of the tongue.

frenulum

[fren-yuh-luhm] / ˈfrɛn jə ləm /

Anatomy, Zoology. a small frenum.
Entomology. a strong spine or group of bristles on the hindwing of many butterflies and moths, projecting beneath the forewing and serving to hold the two wings together in flight.