There are some stickies on exactly this subject. Like the Brain and Addiction one. You'd probably enjoy that.
Whatever addiction actually is is far less important for those with a history of problematic drug use than their actual history (especially if it is ongoing) of problematic drug use. Getting wrapped up in that question is interesting, but more often than not pretty fruitless - especially in the moment. For someone with a history of problematic drug use, it's only really productive in hindsight.
Plus, it's not exactly a self-evident subject. For you addiction is almost certainly going to have a different (if not radically different) character than me or the next person - take someone with problems eating, using alcohol and using heroin (e.g. an addiction to something all humans need to do, an addiction to something socially acceptable, and an addiction to something taboo). There will be some things in common. These are probably what you're getting at. Parsing them is not the simplest thing in the world.
Modern medical/accepted definitions all address addiction as problematic drug use in the past - e.g. a problematic history of drug use - and how it impedes one's functioning (as in a psychiatric disorder; over the last decade we've seen more cognitive and neurosciences approaches, but they follow from the definitions that became popularized in the 1980s). I personally find the sociological components of addiction much more pressing, as well as the broader philosophical implications any definition or understand of addiction inevitably has.
When I first saw this thread I was a little put off reading the opening post. Most people seem to understand Addiction as pretty much a mirror images of their own experience of addiction - they have a hard time understanding how it can be radically different for even very similar people under very similar circumstances. Extrapolating too much from anecdotal evidence, especially when it comes to addiction, can be particularly dangerous given the overwhelmingly "self-help" or non-professional nature of addiction treatment (particularly in the US).
So I was a little nonplussed at first. But it is an interesting subject which I too continue to explore. There are some really good books about this out there, from Chasing the Scream to The Birth of Heroin and the Demonization of the Dope Fiend to more scholarly works like Creating the American Junkie: Addiction Research in the Classic Era of Narcotic Control (on of my favorites). There are also good research papers out there, like the International Journal of Drug Policy (for instance,
http://www.ijdp.org/article/S0955-3959(13)00010-8/abstract).