Tit slit
The very slight and sexually arousing slit that appears when a woman is barely exposing just enough of her cleavage (also known as breasts, "titties", "honkers", "jugs") so as to entice straight men and lesbians. Can often be far more titilating (pun fully intended) than a woman exposing all her top half immediately.
"She was a saucy minx, but not a full on slapper. You could see just the right amount of tit slit"