Loli
The term loli is used on 4chan to refer to girls who are (or appear to be) pre-pubescent, or at most in early adolescence. The word is derived from lolicon, a Japanese term which is itself an abbreviation of Lolita complex, used in Japan to refer to pedophilia. In the west, the term lolicon is instead used to describe (especially Japanese) drawn and animated pornography involving such apparently underage characters and those who peruse it. Though the term lolicon is used almost exclusively in a sexual context, within the anime fandom the term loli is often used to refer to any girl that meets the aforementioned criteria, regardless of sexualisation or lack thereof. An well-known example of such a loli is Yotsuba, the mascot of 4chan.
Some people contend that using the term loli to describe a character always implies a sexual attraction on the part of the speaker, which is of course vigorously denied by those who use it. There are some who claim that a character's given age can exempt them from loli status (for example, the character Konata from Lucky Star, who is 18 years old but also an extremely late developer such that she appears to be roughly eleven or twelve); others generally hold that these people are merely trying to justify their attraction to such a character without admitting they are paedophiles, since there doesn't appear to be any other reason to try and make this distinction.
Related to this is the slang otaku term "moe" (萌え, pronounced [mo.e]), which literally means "budding," and a pun on "burning" with passion or light or life. This originally described a sort of protective instinct invoked by certain characters. These days it's either used by fools with white-knight syndrome to describe girls that are so young and cute you want to take care of them, or used by filthy pedos to masquerading as the former.
The rule 63 version of Loli is Shota, which is typically preferred by either overly maternalistic women or very, very creepy men.
