![1girl aiming_at_viewer alternate_hairstyle buttoniris closed_mouth collared_shirt commentary crossdressing english_commentary finger_on_trigger grey_pants gun highres holding holding_gun holding_weapon jitome looking_at_viewer mirror multiple_views original pants pink_hair red_eyes reflection shirt short_hair simple_background suspenders sweatdrop thought_bubble tomboy two_(buttoniris) very_short_hair weapon white_background white_shirt wing_collar rating:s score:6 post #3892664](/cdn_image/preview/27/07/27076a151b39182021d4e8bfd014679b.jpg)
Essentially refers to a girl that behaves like a boy. Girls who are tomboys tend to be very athletic and sporty.
They almost always dress like boys and in anime frequently have short and/or spiky hair and can often be seen wearing tight cycling shorts. Examples would be Kagura (Azumanga Daiou), Arisawa Tatsuki (Bleach) and for a videogame example, Makoto (Street Fighter III: 3rd Strike).
Similar to the androgynous tag, but differs in that androgynous apply to both men and women, referring to possessing physical traits associated with both sexes (not usually in terms of genitals, but rather facial features and build). An androgynous person wouldn't necessarily be a tomboy at all (they may actually be male anyway) but a tomboy is somewhat androgynous.
Note: Just having short hair is not enough to count as a tomboy. The girl in question has to dress and act in a masculine manner.