Etymology
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cleavage (n.)

1805, in geology and mineralogy, "tendency (of rocks or gems) to break cleanly along natural fissures," from cleave (v.1) + -age. General meaning "action or state of cleaving or being cleft" is from 1867.

The sense of "cleft between a woman's breasts in low-cut clothing" is first recorded 1946, defined in a "Time" magazine article [Aug. 5] as the "Johnston Office trade term for the shadowed depression dividing an actress' bosom into two distinct sections;" traditionally first used in this sense by U.S. publicist Joseph I. Breen (1888-1965), head of the Production Code Administration (replaced 1945 by Eric Johnston), enforcers of Hollywood self-censorship, in reference to Jane Russell's costumes and poses in "The Outlaw."

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Definitions of cleavage

cleavage (n.)
the state of being split or cleft;
there was a cleavage between the liberal and conservative members
cleavage (n.)
the breaking of a chemical bond in a molecule resulting in smaller molecules;
cleavage (n.)
(embryology) the repeated division of a fertilised ovum;
Synonyms: segmentation
cleavage (n.)
the line formed by a groove between two parts (especially the separation between a woman's breasts);
cleavage (n.)
the act of cleaving or splitting;
From wordnet.princeton.edu