In martial arts, a short spirited yell with several functions.

1. It scares the hell out of your opponent. Whether it terrifies them into running away or just causes them to flinch, the kia is an effective psychological weapon.

2. It gives you an adrenaline rush. Adrenaline is a good thing to have in a fight and screaming seems to stimulate those tiny endocrine glands quite nicely.

3. It gives your strikes more power. I'm not sure how much I beleive this one, but it has been said that the force of yoru diaphragm rapidly contracting adds to the force of your strike.

The kia is often mispronounced, "hy-ya!" Whichever you like, although using hy-ya tends to make you look ignorant.