The universal math constant where every frequent female facebook, myspace, ect... user has at least one outstanding picture of their respective cleavage. Cases which include more than one include: the slut in your 3rd period gym class, the beach photos from spring break, and of course the ever loved low cut shirts.
Note: often along with said cleavage photo, an album with possibly more photos of similar exposure may be found that most likely contain side boob, under boob, and/or camel toe in addition.