Going off the available evidence of this particular discussion, we're far from having a common working definition in this instance... Part of the reason for this disparity is that the huge variety of legal definitions are actually clouding the colloquial usage that we'd be much more likely to (almost) universally accept. In essence, self defence is easy to understand - it's the action of defending oneself. Simple. But then as soon as you start a discussion on the subject in detail, problems arise with whether you're defending yourself or defending someone else, whether it's actually classifiable as self defence if you didn't comply with the duty to retreat and/or stand your ground (and then an argument about those duties not applying everywhere), whether it's transformed into a consensual fight because you hit back/hit first, how fast and how far you are able to run and whether your assailant is armed with a cheeky cabernet. I can't see a way to move this forward myself.