The visible by-product when a woman wears a bra/corset/top that pushes her breasts together.
Sometimes used purposly as an attractiontechnique when tartish girls want shallow guys attentions.
guys in club: "God, look at her cleavage in that top!"
Girl (upon hearing comment): " Hi, wanna buy me a drink *stupid high pitched giggle*"
Cleavage is the most important thing in the world for most men or they just don't care. Also you can compare it to the sun. You can't stare at it to long, it's to risky.