Back cleavage refers to when a woman's low cut shirt or dress reveals a slim, sexy back.
"That back of that girls dress is ridiculous! The plunging back line looks nice."
"Agreed, she has some nice back cleavage!"
when a fat lady (particularly old), wears a swimsuit or bra that is too tight in the back and it squishes the back fat together to give the illusion that there are boobs on their back.
Lifeguard 1: "that old lady is large"
Lifeguard 2: "yeah man, look at her back cleavage, it's impressive."