What is your definition of self defense?

[Mr Uncle Leroy]

there was the situation were a 46 year old man who lost his life due to police thinking they were threatened...

or the situation in the South Africa mine...were police upon fire on miners...in that situation police should have had riot gear, etc

also there is Zimmerman situation with Trayvon, etc

in addition to the issues and loss around mistaken perception, or in someones mind feelings on needing to protect self.

what is your definition of self defense?