Google's leaked documents defining spam state: When trying to decide if a page is Spam, it is helpful to ask yourself this question: if I remove the scraped (copied) content, the ads, and the links to other pages, is there anything of value left? if the answer is no, the page is probably Spam.
Yahoo defines spam as "pages created deliberately to trick the search engine into offering inappropriate, redundant, or poor-quality search results."
Bottom line: A spam site is one which uses techniques in a determined manner to subvert the search engine's algorithms, to artificially inflate their search engine rankings. (http://searchenginewatch.com)