Beneath Ass Tan Lines. This acronym is used to describe the tan lines girls get when they use the tanning bed and get the tan lines underneath each cheek which are then visible when they are at the beach, naked, or wearing some skanky shorts. It is usually the adverse effect of too much junk in the trunk. It's not often you see a small booty with BATL's.
(At the beach) Damn dude, look at the BATL's on that girl. She's definitely been in the tanning bed.