Cleavage can mean may things
1.the act of cleaving or splitting
2.the state of being cleft
3.the area between a woman's breast, esp. when revealed by a low-cut neckline
4.a critical division in opinion, beliefs, interests, etc, as leading to opposition between two groups: a growing cleavage between the Conservative and Liberal wings of the party
5.the tendency of cystals, certain minerals, rocks, etc., to break in preferred direction so as to yield more or less smooth surfaces