Cleavage is the low-cut in women's clothing, partially revealing the women's breasts. It is also used with an erotic purpose.
In 1805, in geology and mineralogy, cleavage meant "tendency (of rocks or gems) to break cleanly along natural fissures." General meaning "action or state of cleaving or being cleft" is from 1867.
The sense of "cleft between a woman's breasts in low-cut clothing" is first recorded 1946, defined in a "Time" magazine article.