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.

the thing that you don’t have when you’re phones percentage is over 20

A varied slang term used to substitute the word "Cleavage".