SPAM used to be a straightforward term. It meant unsolicited bulk e-mail. Now, anything unwanted is called SPAM.
This is true for the search engines and the website marketer. For website marketers, some define SPAM as "search positions above mine." Search engines seem to define SPAM according to what affects their bottom line.
The "official" definition.
What SPAM should be:
any attempt to alter or obtain rankings causing end users to complain about the quality of the search engine results.
What SPAM has become:
any attempt to alter or obtain rankings not directly involving cash payment to said search engine.
Yeterday's spammers are today's "partners".
The "convenient" definition.
Here a few examples how search engines change the meaning of SPAM to suit their purposes.
SPAM Example 1.
Company A operates an extremely large dynamic online store. They want search engine traffic, so they hire a SEO consultant. A few months later, their programmers finish work on a new system removing all spider roadblocks.
Search Engine Spider A appears and crawls the site. Over time, several thousand pages get indexed. Someone from Search Engine A notices all the pages from the site. They decide the site is "over-represented" and removes the majority of pages.
Shortly after Company A notices the drop in traffic, a sales rep from Search Engine A contacts them. The sales rep tells them about a program which gets all their "deep content" included. They sign up and begin paying $.25 per click. Suddenly, the same bunch of pages goes from being "over represented" (SPAM) to "valued content."
SPAM Example 2.
Company B pays a design firm to build a cutting-edge site with all the latest bells and whistles. After the fact, they realize the way they are presenting the content makes it impossible for search engines to index their pages. So, they hire a consultant.
Unwilling to completely redo the site they just completed, they instead decide to use IP delivery. This provides spiders with indexable descriptive content.
Search Engine B discovers what they are doing. They kick them out. Their spider gets content different from what a human sees. It is the ultimate form of SPAM.
A week later, Company B signs up for Search Engine B's new program. They begin delivering the exact same content, via an XML feed. All of a sudden, Company B goes from being a spammer to being "trusted."
SPAM Example 3.
Search Engine C actively tracks those who SPAM them. This leads to several large SEO firms getting complete C blocks banned from Search Engine C's database.
The number one violation committed by these companies is cloaking hundreds of pages which deliver all users to one page. Search Engine C doesn't like this practice, because users get upset when they click on what appears to be a different listing. They end up on a page they've already visited.
A few months later, Search Engine C announces they have some new partners. Most of the new partners are SEO firms employing the same type of methodologies used by many of the companies on Search Engine C's SPAM list. The only difference seems to be the partners are paying Search Engine C, and the companies on the list are not.
Conclusion.
Depending on what you are trying to do, the definition of SPAM seems to change.