So here's the first one I find: "the defense of one's person or interests, especially through the use of physical force, which is permitted in certain cases as an answer to a charge of violent crime." That leaves a lot of undefined borders, because it doesn't really delineate what "one's...interests" would include. Here's the second one I found: "the act of defending oneself, one's property, or a close relative." That already has a different element (protecting another person), unless we consider that within "one's...interests". That's what I mean. Two people can use that first definition, agree that it's correct, and still disagree on the boundaries of it. Because it's conceptual (as opposed to the legal definition, where the boundaries tend to be better defined by case law - though that still varies somewhat by jurisdiction).