Cleavage (noun)

1. A split or separation, especially in a mineral or rock.
2. The space or opening between a woman's breasts, especially when revealed by a low-cut garment.

Origin:

From middle english, meaning "a cleft or split".

Examples:

1. The rock had a distinct cleavage.
2. The mineral had a perfect cleavage.
3. She wore a dress with a deep cleavage.
4. The cleavage in the rock made it easy to break.
5. The cleavage in her dress was quite revealing.