Cats hiss at people and other animals, including cats, as a show of aggression. This generally occurs because a cat feels like it is cornered and has no means of escaping or the cat is provoked. Aggression can also be invoked by pain, punishment or fear.
Hissing is not the only sign of aggression. There are other ways that cats show aggression, including facial expressions and body posture. Aggression may be expressed by staring, stalking, swatting, showing their teeth and pouncing. Other behaviors that indicate aggression in cats include back arching, standing their tails straight up, raising the hair on their back, dilated pupils and pulling back their ears. Cats may also rub their chins against items or people to mark their territories or they may spray an area to claim it as their own. Drawing in of the limbs to hide the belly and neck also shows aggression, and an obvious aggressive sign is attacking with the teeth and claws.
Aggression can be caused by unhealthy environments, including crowded catteries, shelters and caged living. Cats that are not accustomed to being around children may also hiss out of fear or because children are rough with them. Hissing by a mother cat is her way of protecting her kittens.