A girl (or sometimes a boy, like royo) that has unnaturally big breasts for his/her age.
Usually can be spotted during sportactivities like running. These people have massive hormone-machines inside them.
Dude #1: Hey check out that boobtron!
Dude #2: Wow, she sure is wiggling with the twins
Dude #3: Ya she shouldnt be running
-After that everyone is quietly looking at the boobtron-