User blog comment:HiddenVale/Enlighten me, please/@comment-452-20121001030909

Spam is a catch-all term for anything unwanted.

The origin of the term is from Monty Python's Spam sketch, where a group of Vikings are constantly singing about Spam (The canned meat product)

Possible uses of spam include:
 * Flooding
 * Advertising
 * Inserting gibberish
 * Cross-posting

Most people's definition of the word is vague, so the only thing you can "rely" on is that "spam" means something unwanted.