a small fold or ridge of tissue that supports or checks 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.
It's important to call it Frenulum Linguae Frenulum itself just stands for very thin skin. Guys also have it on their best part if you get what I mean And there is many other "Frenulum blablabla" so always remember to write the specific location to it.