Tweener a.k.a. Perineum
This term refers to the small patch of skin between a woman's thighs to the right and left sides, and between the bottom of the vaginal opening to the front and the anus to the rear. The tweener is an erogenous zone for most women, and if touched or licked gently, can produce a state of extreme arousal. It can be accessed from many positions, but most easily when the woman is lying on her back, and her partner lifts her buttocks towards his or her face. Doggy sex position also works for this.
Men also have a tweener, located between the base of the scrotum and the anus. Some men also find this to be an erogenous zone, and like to be touched or licked there as well.
The term "tweener" originates from the fact that this patch of skin is be-"tween" the anus and the genitals.