The "spirited shout" that is heard in many martial arts.
The voice is essential in self-protection skills. When violence is occurring you'll never hear a police officer meekly asking "I say, dear chap, would you kindly mind putting that knife down?" Instead you hear "PUT THE KNIFE DOWN. NOOOWWWW" Consider also that some sportspeople such as weight lifters or throwers (javelin, shotput) will vocalise as they endeavour to express as much energy or force as they're able.
And so it is for us. Let's explore the kiai.
気 Ki - energy, spirit or lifeforce
合 ai - to join or to harmonise.
So it is a joining of energy. Consider the inverse of kiai - aiki - as seen in Aiki-Do, the way of harmonious spirit. The kiai should enable us to find and express our spirit, to give us courage, to frighten our attackers.
It should be personal. The noise you use should be personal, natural, from the belly and be single-syllable. It should not be a "word." It may be something like "ha," "hup," "hap," "oss," "aiy" or many other sounds. What it should never be is the word "kiai." Imagine trying to hit your attacker with every bit of strength and spirit that you have whilst declaring "joining of energy!" It just doesn't work and sounds stupid, right? It also takes too much breath and voice control to make this word of two syllables.
New students often find this unnatural but it should become a part of all martial artists' arsenal and performance.