fre·num 
(frēnəm)
(frēnəm)n. pl. fre·nums or fre·na (-nə)
A membranous fold of skin or mucous membrane that supports or restricts the movement of a part or organ, such as the small band of tissue that connects the underside of the tongue to the floor of the mouth.
[Latin frēnum, bridle, from frendere, to grind; see ghrendh- in the Appendix of Indo-European roots.]