Ki-ai is defined as "spirit meeting" or the "martial art shout". The Ki-ai is used to clear the mind, give confidence, and also frighten an opponent. It tightens up the body, especially in the abdominal region, thereby protecting one from blows. The Ki-ai is considered the active, verbal side of the mind while Ai-ki, its partner, is considered the passive, silent side. When written, they are in fact composed of the same two kanji characters in reverse order.

It was believed that some martial arts masters in feudal Japan were able to use the martial shout to either injure or heal another. This was known as Kiai-jutsu, "the art of ki-ai". Kiai-jutsu masters on one hand could stun a person, or even animals, with a powerful yell, and on the other revive unconscious people or perform such feats as stopping nosebleeds with a Ki-ai.

When the Ki-ai is uttered by a martial artist, the vibration of the sound may momentarily paralyze the opponent's functioning and render him or her more susceptible to an attack. The Ki-ai enables the person carrying out a violent movement to purify the mind of extraneous thoughts, leaving simply the pure energy (Ki) which causes the person to act, and confers upon them all its intensity.

There are certain martial arts masters that maintain that there are three or four kinds of Ki-ai: low and weightily at moments of action, high and piercing with a cry of victory, normal for the purposes of resuscitation, and silent (Kensei) in certain meditation exercises.