Cleavage

the act of cleaving or splitting.
the state of being cleft.
the area between a woman’s breasts, especially when revealed by a low-cut neckline.
a critical division in opinion, beliefs, interests, etc., as leading to opposition between two groups:
a growing cleavage between the Conservative and Liberal wings of the party.
the tendency of crystals, certain minerals, rocks, etc., to break in preferred directions so as to yield more or less smooth surfaces (cleavage planes)
Embryology. the total or partial division of the egg into smaller cells or blastomeres.
Also called scission. Chemistry. the breaking down of a molecule or compound into simpler structures.
Contemporary Examples

My Anger at Obama Meghan McCain November 28, 2009
Why Do Men Send Dick Pictures? The Schlong Has No Chance Aurora Snow July 25, 2013
The Week in Death: Clarissa Dickson Wright, One of ‘Two Fat Ladies’ The Telegraph March 21, 2014
And The Escort of The Year Is… Backstage at The Sex Oscars Scott Bixby March 23, 2014
Helen Gurley Brown’s Fashion Sense: the Power of Cleavage Robin Givhan August 13, 2012

Historical Examples

Out To Win Coningsby Dawson
Fragments of science, V. 1-2 John Tyndall
On Digestive Proteolysis R. H. Chittenden
Fragments of science, V. 1-2 John Tyndall
Geology William J. Miller

noun
(informal) the separation between a woman’s breasts, esp as revealed by a low-cut dress
a division or split
(of crystals) the act of splitting or the tendency to split along definite planes so as to yield smooth surfaces
(embryol) Also called segmentation. (in animals) the repeated division of a fertilized ovum into a solid ball of cells (a morula), which later becomes hollow (a blastula)
the breaking of a chemical bond in a molecule to give smaller molecules or radicals
(geology) the natural splitting of certain rocks, or minerals such as slates, or micas along the planes of weakness
n.

cleavage
(klē’vĭj)