Cleavage is the space between the breasts. The smaller the cleavage the better - in that less space is better. This is usually done by pressing the breasts together.
By the way, "cleave" is an unusual word. It has two meanings, which are rough opposites. You can cleave something and break it into two pieces; or you can cleave together and unite with your friends and allies.
Cleavage is also an important distinguishing property of minerals, and is a key test in physical geology.
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