Bokukko

Bokukko (ボクっ娘) Also known as “Bokko”, it is a Japanese term for a girl that acts like a boy. (lit. a tomboy) The term originated from the fact that “Bokukko” tend to use “boku” (masculine form of I) rather than “watashi” (neutral form of I) when referring to themselves.
Bokukko is combination of the words “boku” 僕/ボク and “musume” 娘 (read as “ko”), which literally translates to “I (masculine form of I) girl”.[sc:BR]
A Bokukko is a female character who uses male pronouns. This term arises from the Japanese Pronoun boku, primarily used by boys and young men, and “ko”, a feminine suffix. Extreme tomboys may prefer the pronoun ore, normally used by Hot-Blooded young men. (This, incidentally, is an example of how Japanese writing differs from Japanese speech; although feminine speech patterns have by and large become more “neutral” over the years, it would still be quite unusual and out of place for a woman to refer to herself with a masculine pronoun.)
Like many of the “-kko” terms, this has its place in female archetypes in anime or Japanese Video Games. Most, but not all, bokukko are tomboys, and not all tomboys are bokukko, as the term revolves solely around the use of the pronoun “boku”. Sometimes, it can just simply indicate that the user is a Plucky Girl.
While the use of boku most often signals tomboyishness, it can sometimes signal some other situation, such as not knowing correct societal behavior, or lacking polite speech. Sometimes it can be used to keep a characters gender obscured—is she a boyish girl, or a girlish boy?
Whenever a Bokukko that primarily uses the male pronoun permanently switches to primarily using the female pronoun, it’s a plot point. When the show is dubbed, however, this will invariably lead to a Dub-Induced Plot Hole due to the lack of gender-specific first person pronouns.
Also, a Bokukko character will usually be addressed with the “-kun” honorific.
Although none of this has to be reflected in her appearance, the bokukko is usually either flat-chested or extremely well-endowed. A Dark-Skinned Redhead is likely to be a bokukko, but it’s not guaranteed.[sc:BR]
A Japanese term used to name those girls that address themselves as “boku”, a boyish way to address oneself, it’s one kind of “moe” if you wonder.
One of the most famous “bokukko” examples is probably Ayu-chan from Kanon, a popular visual novel in Japan. Via: utaite, tvtropes, urbandictionary