a fold of skin or mucous membrane that checks or controls the movement of an organ or part, as the fold under the tongue
See frenum in American Heritage Dictionary 4
(frēˈnəm)
nounpl.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.