Cleavage is a word for a kind of splitting or separation: it's mainly used for the split between a woman's breasts.
We can talk about many kinds of cleavage, such as the splitting of a cell or a hunk of wood. Political movements will often have a cleavage, as differences of opinion create factions. This word is interesting because cleavage is not only a separation, but it can mean a bringing together. When you think of the cleavage created by a woman's breasts — stop staring, by the way — you can see the separation and bringing together very well.