short for the word lesbian...or a female who loves the poon-tang of another female. you may also use this term in reference to an individual in a large group of of female friends who claim they are all straight but really are not. bian sex toys can be the most fucked up things you've ever seen in your life.
Girl 1: Do you not want to hang out with us cause we're all bians?
Boy 1: Um, no...Wait what the hell is a bian?
Girl 1: A lesbo..
Boy 1: HAHAHAHAHAHA!
Girl 2: Hey! Where's our lead pipe toy?