cleavage

American Heritage® Dictionary of the English Language, Fourth Edition

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

Century Dictionary and Cyclopedia

1. n. The act of cleaving or splitting, or the state of being cloven.
2. n. 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. The cleavage shows the direction in which the force of cohesion is least. (Compare parting.) It is defined as perfect or eminent, imperfect, interrupted, etc., according to the ease with which the fracture takes place, and the smoothness of the resulting surface; also cubic, octahedral, rhombohedral, prismatic, basal, etc., according to the direction of the fracture.
3. n. In geology, the property possessed by certain rocks of being easily split or divided into thin layers. It is chiefly the argillaceous rocks in which cleavage is highly developed, and it seems to be the result of metamorphism combined with pressure. The cleavage of rooflng-slate is the best illustration of this structure.
4. n. In embryology, segmentation, specifically of the vitellus: distinctively called egg-cleavage or yolk-cleavage. See segmentation.

Wiktionary

1. n. chemistry The splitting of a large molecule into smaller ones.

GNU Webster's 1913

1. n. The act of cleaving or splitting.
2. n. (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.
3. n. (Geol.) Division into laminæ, like slate, with the lamination not necessarily parallel to the plane of deposition; -- usually produced by pressure.

WordNet 3.0

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