Cowgirl is a sexual position with the woman riding on top of the man or woman. With a male partner or a woman using a strap-on it allows for deep penetration.
The name derives from the image of the woman "riding" her partner like a cowboy rides a bucking horse.
It is often seen as a position allowing for female domination of her partner. Some submissive women dislike being in any on-top position because they link it with dominance and therefore feel very uncomfortable. However, many dominant men feel quite comfortable ordering their lover to pleasure them in this way: orgasm can more easily be delayed and can therefore be more intense; it can be visually very pleasing as much of the woman is exposed; it allows the man to feel most parts of the woman's body.
Because of the association with female dominance, it is one of the positions promoted in feminist journals and lectures.
The associated reverse cowgirl position, in which the woman faces the other way, is not particularly practical, but is much used in porn films because it works well from a cinematographic point of view.