A girl who displays boyish behaviors. Girls who are tomboys tend to be very athletic and sporty.
They almost always dress like boys and frequently have short and/or spiky hair and can often be seen wearing tight cycling shorts. Examples include Kagura, Arisawa Tatsuki and Makoto.
Not to be confused with reverse trap, which refers to a girl with an androgynous or masculine physical appearance regardless of personality. An androgynous girl may not necessarily act like a tomboy and a tomboy isn't required to look androgynous. Though there is a recognizable overlap between the two. Crossdressing does not automatically make a girl a "tomboy" either.
Note: Just having short hair is not enough to count as a tomboy. The girl in question has to act in a masculine manner.