cleavage

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The act of splitting or cleaving.
• noun The state of being split or cleft; a fissure or division.
• noun Mineralogy The splitting or tendency to split of a crystallized substance along definite crystalline planes, yielding smooth surfaces.
• noun The series of mitotic cell divisions that produces a blastula from a fertilized ovum. It is the basis of the multicellularity of complex organisms.
• noun Any single cell division in such a series.
• noun Chemistry The splitting of a complex molecule, such as a polysaccharide, into simpler molecules.
• noun Informal The hollow between a woman's breasts, especially as revealed by a low neckline.

from The Century Dictionary.

• noun The act of cleaving or splitting, or the state of being cloven.
• noun In mineralogy, the property possessed by many crystallized minerals of breaking readily in one or more directions, by which means surfaces more or less smooth are obtained.
• noun In geology, the property possessed by certain rocks of being easily split or divided into thin layers.
• noun In embryology, segmentation, specifically of the vitellus: distinctively called egg-cleavage or yolk-cleavage. See segmentation.

from the GNU version of the Collaborative International Dictionary of English.

• noun The act of cleaving or splitting.
• noun (Crystallog.) The quality possessed by many crystallized substances of splitting readily in one or more definite directions, in which the cohesive attraction is a minimum, affording more or less smooth surfaces; the direction of the dividing plane; a fragment obtained by cleaving, as of a diamond. See Parting.
• noun (Geol.) Division into laminæ, like slate, with the lamination not necessarily parallel to the plane of deposition; -- usually produced by pressure.
• noun cleavage parallel to the base of a crystal, or to the plane of the lateral axes.
• noun (Biol.) multiplication of cells by fission. See Segmentation.
• noun cleavage parallel to the faces of a cube.
• noun cleavage parallel to ta diagonal plane.
• noun (Biol.) See Segmentation.
• noun cleavage parallel to the lateral planes.
• noun cleavage parallel to the faces of an octahedron, dodecahedron, or rhombohedron.
• noun cleavage parallel to a vertical prism.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun chemistry The splitting of a large molecule into smaller ones.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the line formed by a groove between two parts (especially the separation between a woman's breasts)
• noun the breaking of a chemical bond in a molecule resulting in smaller molecules
• noun the act of cleaving or splitting
• noun (embryology) the repeated division of a fertilised ovum
• noun the state of being split or cleft