Cleavage literally means the gap between a woman's breasts. It is generally used to mean the sight of this gap, or of the breasts themselves, as revealed by a low-cut garment. The neckline of a garment that exposes cleavage is known as décolletage.
Perhaps not surprisingly, most fetish fashion designers design many of their women's garments to emphasise the wearer's cleavage.
The term is also used to describe a glimpse of the top of the buttocks (of a man or woman) over low-cut trousers; this is also called rear cleavage or buttock cleavage. In a non-sexual context (expressing more disgust than arousal), buttock cleavage can be known as "plumber's crack" or "builder's bum".