A Cleavage Window is a cut in a woman't top that shows off parts of her breasts. It doesn't matter the shape or the size (of either the Cleavage Window or the breasts) as long as the window is completely enclosed. The rest of the outfit doesn't matter either; this can appear on a bathing suit, a leotard, a minidress, or a formal dress.
See also Most Common Superpower. Often a part of a Stripperific costume. Compare to Absolute Cleavage and Impossibly Low Neckline.