Basically just cleaveage. like a buttcrack only in the front and on your chest formed from 2 large boobs pressed together.
"A slippery coconut noodle just slid down my boob crack!"
"wow, she is showing a lot of boob crack"
"my dang titties are too small for me to have a boob crack"
person a: what are you staring at?
person b: your amazing boob crack!
Boob crack: a great place to carry a cell phone.