Well, since this is the SEO forum . . .
Spam is an acronym - it stands for
Sites
Positioned
Above
Mine
But in all seriousness, when it comes to search engine spam, I see that as any web page that uses
deceptive practices, either in how it appears to search engine spiders and/or consumers, or in how it gets its rankings.
As a for-instance, cloaking and showing bots (or better yet, all Mountain View, California IP addresses
) one page and consumers another is definitely spam. Using hidden text, hidden links, or collecting links through questionable means (like a visitor counter that you have available for free, but that also contains a small, anchor text optimized link to your target page) is also spam.
Also, hacking another person's site and hiding pages on their domain, whether with the intention to grab rankings/traffic, or just to dilute their site's "theme" in Google's eyes, and kill their rankings, is most certainly spam.