cleavage

Etymology

cleave + -age

noun

1. The act of cleaving or the state of being cleft.
   Mineral cleavage is a property of cohesion. The cleavage of the particles themselves and its attitude with reference to the plane of the greatest and mean dimensional axes help to determine the perfection of rock cleavage. The more nearly parallel to the greater dimensions of the particle, and the better the mineral cleavage, the better the rock cleavage produced. 1905, Charles Kenneth Leith, “The Observed Relation of Secondary Rock Cleavage to the Parallel Arrangement of Mineral Particles”, in Rock Cleavage (United States Geological Survey Bulletin; no. 239), Washington, D.C.: Government Printing Office, →OCLC, page 51
   It will become apparent from the following discussion that some of the various structures which are grouped together as cleavage may correspond to a schistosity and/or foliation. Accordingly, Wilson defines rock cleavage (and schistosity) as a planar structure, usually distinct from stratification, which permits rocks to be fractured or cleaved into thin slices. Dennis (1977) defines cleavage as a set of closely spaced secondary, planar, parallel fabric elements that impart mechanical anisotropy to a rock without apparent loss of cohesion. This latter definition appears more precise; however, as we shall see, it presents many difficulties in dealing with those forms of cleavage which are mainly defined by foliation or by fractures. 1990, N[eville] J. Price, J[ohn] W. Cosgrove, “Rock Cleavage and Other Tectonic Fabrics”, in Analysis of Geological Structures, Cambridge: Cambridge University Press, page 434
2. The hollow or separation between a woman's breasts, especially as revealed by a low neckline.
   Low-cut Restoration costumes worn by the Misses Lockwood and Roc (see cut) display too much "cleavage" (Johnston Office trade term for the shadowed depression dividing an actress' bosom into two distinct sections). 5 August 1946, “Cinema: Cleavage and the Code”, in Time
   Wendy thought that her therapist was looking down her cleavage and said so. The therapist was mortified and chiefly felt guilty, unsure whether she had been looking down her patient's cleavage or not. On a reflex she assured Wendy that she was not looking down her cleavage and the matter appeared to rest. 2003, Chess Denman, Sexuality: A Biopsychosocial Approach, Basingstoke, Hampshire: Palgrave Macmillan
   All this time I kept my gaze fixed on hers, an enormously difficult task given the gravitational pull exerted by her cleavage. While I was critical of many things when it came to so-called Western civilization, cleavage was not one of them. 2015, Viet Thanh Nguyen, The Sympathizer, New York, N.Y.: Grove Press
3. (by extension) Any similar separation between two body parts, such as the buttocks or toes.
   Half an hour later, she was staring at a large bottom-cleavage, which was protruding from under the bed. / 'Ohhh Kayyy. Here we go. This is basically your problem.' / Olivia took a few steps back as the bottom-cleavage started wriggling out towards her. Connor the counter-surveillance expert pulled himself awkwardly to his knees, […] 2003, Helen Fielding, Olivia Joules and the Overactive Imagination, London: Picador, page 95
   Brenda lifted the top off a shoebox and dug beneath the tissue paper, unearthing a two-toned, turquoise-and-cream, high-heeled pump. "This is one of my favorites," she said, balancing it in the palm of her hand. "I love shoes that show toe cleavage." / "Toe cleavage?" / Brenda kicked off a leather sandal, slipped her foot into the pump, and pointed to the baseline of her toes peeking through. "That's toe cleavage!" 2014, Bridgett M. Davis, Into the Go-Slow, New York, N.Y.: The Feminist Press at the City University of New York
4. (biology) The repeated division of a cell into daughter cells after mitosis.
   [C]itellate annelids (leeches + oligochaetes) provide solid ground for comparative analyses, because they form a monophyletic group within the paraphyletic polychaetes[…]. […] The unequal cleavage of the zygote followed by the unequal cleavage of the CD cell is critical for D-quadrant specification since the cytoplasmic determinants present in teloplasm are segregated to the larger CD cell at first asymmetric cleavage and then to the larger D cell at the second asymmetric cleavage[…]. 2008 spring, Dierdre Colleen Lyons, “Mechanisms Controlling the Asymmetric Second Cleavage”, in Mechanisms Controlling the Asymmetric Second Cleavage in the Helobdella Embryo (unpublished Ph.D. in Molecular and Cell Biology dissertation), Berkeley, Calif.: University of California, Berkeley, →OCLC, page 16
5. (chemistry) The splitting of a large molecule into smaller ones.
   Carotenoid cleavage oxygenases (CCOs) are a class of enzymes that oxidatively cleave carotenoids into apocarotenoids. Carotenoid cleavage oxygenases have been identified in plants and animals and produce a wide range of cleavage products. […] Two of the three enzymes showed cleavage of β, β-carotene at the 9,10 and 15,15' position, respectively. 2006, E. K. Marasco, K. Vay, C. Schmidt-Dannert, “Identification of Carotenoid Cleavage Oxygenases from Nostoc sp. PCC7120 with Different Cleavage Activities”, in Journal of Biological Chemistry, volume 42, abstract, page 281
6. (mineralogy) The tendency of a crystal to split along specific planes.
   Notice that cleavage has nothing to do with imperfections in the crystal. Indeed, if you try cleaving an imperfect crystal, you may find that it cannot be cleaved as well as a perfect one. The cleavage is the result of the orderliness of the atomic arrangement. No wonder imperfections, which disturb the orderliness, disturb the cleavage as well. You will also note that the cleavage directions are obedient to the symmetry of the crystal. 1982, Alan Holden, Phylis Morrison, “Cleaving and Gliding Crystals”, in Crystals and Crystal Growing, Cambridge, Mass., London: MIT Press, page 202
7. (politics) The division of voters into voting blocs.
   A voting bloc in Congress can represent a long-standing alliance or can be created anew as a result of the increased saliency of a particular issue, such as war or a severe economic downturn. A division between voting blocs is called a cleavage and because voting blocs are impermanent, dominant cleavages are always changing. 2011, Scot Schraufnagel, “Voting bloc”, in Historical Dictionary of the U.S. Congress, Lanham, Md.: Scarecrow Press, page 235