Cleavage?

We had a discussion during study hall about what exactly cleavage was.
The question arose when the mistake was made of one of my friends calling her boob cleavage and so we progress. Another friend then asks if she'd been calling her boobs the wrong thing her whole life in her famous southern belle accent where she weaves intricate stories about sleeping with the little black boy working on her plantation. It's hilarious. But I'm off topic.
I came into the conversation now, having been listening whilst working on the World History assignment due in two hours.
After explaining my point, we all agreed that I was right in saying that cleavage is nothing more than an imaginary line, like the Equator. But then it was brought up that the equator cuts the earth into two horizontal halves (not in those words, more like hand gestures indicating the horizontal way), so my former ruling was changed to "Cleavage is an imaginary line that is the equivalent of the Prime Meridian."

Just thought I'd clear up any confusion or debate about what cleavage was.
Thank you for your time.

Erro.