cleav·age 
(klēvĭj)
(klēvĭj)n.
1. The act of splitting or cleaving.
2. The state of being split or cleft; a fissure or division.
3. Mineralogy The splitting or tendency to split of a crystallized substance along definite crystalline planes, yielding smooth surfaces.
4. Embryology
a. 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.
b. Any single cell division in such a series.
5. Chemistry The splitting of a complex molecule, such as a polysaccharide, into simpler molecules.
6. Informal The cleft between a woman's breasts as revealed by a low-cut garment.