Wow, what a huge, huge, huge question. At my level of understanding I am not sure how to answer that but here is a start. First, Marc MacYoung tells you what SD is in chapter three of his book, “In the Name of Self Defense (462 awesome pages long),” starting on page 55 and goes on through to page 91. Take a notice that he writes to the answer and it takes at least thirty-six page - an that is just a basic fundamental explanation. In reality his entire book is describing the answer to “What is self-defense?”
I guess to answer this question my only true answer is to get his book, read it, study it and then continue to seek knowledge as to what is self-defense. Then, start taking on the study of all those fundamental components of SD and study them to gain even more insight into what SD is because as Mr. MacYoung and Mr. Miller have stated along with a lot of other experts or professionals on this subject, “It’s complicated.”
MacYoung, Marc. "In the Name of Self-Defense: What It Costs. When It’s Worth It." Marc MacYoung. 2014.
Goleman, Daniel. "Emotional Intelligence: 10th Anniversary Edition [Kindle Edition]." Bantam. January 11, 2012.
Miller, Rory. "ConCom: Conflict Communications A New Paradigm in Conscious Communication." Amazon Digital Services, Inc. 2014.
Miller, Rory and Kane, Lawrence A. "Scaling Force: Dynamic Decision-making under Threat of Violence." YMAA Publisher. New Hampshire. 2012
Miller, Rory. "Force Decisions: A Citizen's Guide." YMAA Publications. NH. 2012.
Miller, Rory Sgt. "Meditations of Violence: A Comparison of Martial Arts Training & Real World Violence" YMAA Publishing. 2008.
Miller, Rory Sgt. "Facing Violence: Preparing for the Unexpected." YMAA Publishing. 2011.
Morris, Desmond. “Manwatching: A Field Guide to Human Behavior.” Harry N. Abrams. April 1979.
Links to purchase bibliography from Amazon here: http://martialartlibrary.blogspot.com