Just thought I'd check this out. It seems like it is both frenulum and frenum.
From Wiki:
A frenulum (or frenum, plural: frenula or frena, from the Latin frçnulum, "little bridle", the diminutive of frçnum ) is a small fold of tissue that secures or restricts the motion of a mobile organ in the body.
I didn't check wiki when I posted the above, I just checked my PDR. (physicians desk reference) No frenum in that just frenulum. Perhaps it is another neighbor/neighbour thing?