Loli
A loli is a girl who is (or appears to be) physically pre-pubescent or in early adolescence, regardless of 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 terms "Lolita" and "complex". Though the term has pornographic origins, 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.