Cleavage is not just the space between a woman’s breasts, it can mean a lot of things: in biology, it is the repeated division of a cell into daughter cells after mitosis; in chemistry, it is the splitting of a large molecule into smaller ones; and in politics, the division of voters into voting blocs.
English is unique in the way it uses the word ‘cleavage’ for all these different phenomena.
“Breasts are a scandal because they shatter the border between motherhood and sexuality.” – Iris Marion Young
Political systems are characterised by cleavages. These are the metaphorical lines which divide members of the community into different sides. Cleavage lines are mainly founded on values, ethnicity, language or socioeconomic status.
Specific issues, political parties and ideology on the other hand are not the bases for cleavage, but they may well be indicative of the fundamental value conflicts which do constitute the lines of cleavage in a particular system.