What is a Spam?

I've been discussing Spam on this blog for months now.
I've given out, what are according to me the best ways not to be considered as a Spammer.

Now that time passed and that, working at an ESP, I've seen number of very different emails and clients being Spam blocked by ISPs and Spam reported by users.

Now the question that's on my mind currently is "what is the REAL definition of a Spam?"

First thing, as anyone living on the 21st century, I had a look at wikipedia for a definition, there it is

This is mostly a technical definition and a collection of facts.

Unfortunately I think all is not that simple, I think the definition of a Spam varies according to the point of view.

Let's try and see how we would describe a spam according to who we are:
Sender: Well, unless you are one of the Spammers sending Viagra advertising knowing you are spamming millions (billions) of people, you probably think you broadcast legitimate emails.

ESP: Same thing here, unless you are an illegal, underground ESP, you probably trust your clients (you probably have plenty of disclaimers in your contracts as well) and therefore consider you broadcast legitimate emails on behalf of your clients.

End users: We are all end users, so we probably all know what WE would consider as a Spam, basically any email sent to us that we didn't ask for or sent by someone we don't know.

ISPs and Spam filters: I kept those as last (although they should have been placed before the end user in the email chain) because I feel like this is the place where the problem is the less straitforward.
Several reasons for this:
First one is ISP loose money due to Spam, second reason is Spam filters make money out of it.
There's more and more Spam filtering company that make black or grey listed broadcasters pay for unlisting.
We can also mention the emailreg project launched by Barracuda which doesn't make everyone happy.
We also can mention all the certification programs that have flourished over the last years.
ISPs and Spam filter's definition of Spam is some kind of mix of:
Bad content, bad coding, bad keywords, poor sending domain reputation and poor IP reputation.
But even then, some of my clients match 100% of these criterias (I mean in good of course) but they will NEVER achieve 0% spam filtered emails

I guess the real definition of a Spam is an unsolicited email, but it's all a matter of perception after all.