Kiai
Kiai (pronounced "KEY - eye") means spirit harmony. It is the focusing of all one's energy and intention on a single point for a moment in time. It is physically manifested by a yell from the gut, delivered with a strike or receiving a strike. The stereotypical "hi-YAH" associated with karate is a form of kiai.
The miracle of the kiai, in my personal experience, is how it can bring back your energy and air when you're out of breath or have been gutted. I've used it many times in kung fu tests to restore myself.