Definition of a cleavage :
Cleavage, anatomically known as the intermammary cleft or the intermammary sulcus, is the space between a woman's breasts lying over the sternum. In popular culture, it is often defined by garments with low necklines, like ball gowns, evening gowns, lingerie and swimwear that expose or highlight cleavage.
http://en.wikipedia.org/wiki/Cleavage_(breasts)