According to Wikipedia, it is the act of defending oneself, one's property or the well-being of another from physical harm. While the term may define any form of personal defense, it is strongly associated with civilian hand-to-hand defense techniques. Self-defense defines a civilian activity as opposed to Hand-to-hand combat in a military context (from http://en.wikipedia.org/wiki/self-defense)

In actuality it involves much more than mere legal application of force - it also involves the mental training and habitual awareness that keeps you from being on the pointy end of a personal negotiation. It involves Crime Awareness - what does a criminal need to succeed at his job? It also involves Situational Awareness - when the time for diplomacy is over and how to realize it before you are face-down. Lastly real self-defense allows you to sort through actuality and reality while you are experiencing the Adrenal Stress Response (a.k.a. The Fight or Flight response).