Loli
A loli is a girl who is physically pre-pubescent or in early adolescence, regardless of their actual age. The name is derived from "lolicon", a japanese term describing hentai involving such characters and those who peruse it, which is itself a portmanteau of the term "Lolita complex". Though the term has pornographic psychology literary origins (pr0n just picked it up and ran with it and shrinks just liked the book enough to use the name), in chan usage it is used to generally refer to any girl fitting the criteria, without necessarily implying anything sexual. An example of a well known loli is Yotsuba, 4chan's mascot.
Loli are known for their delicious flat chests.