Dictionary.com
Thesaurus.com

frenum vs. frenulum

[free-nuhm] / ˈfri nəm /

noun

Anatomy, Zoology.

PLURAL

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

[fren-yuh-luhm] / ˈfrɛn yə ləm /

noun

PLURAL

frenula
  1. Anatomy, Zoology.  a small frenum.

  2. 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.