Inappropriate commercial message of extremely low value.
The definition of spam is purposely vague because everybody has his or her own definition. As it currently stands, spam is in the eye of the beholder.
The general term spam can apply to messages over many outlets. Some common targets include email inboxes, search engines and discussion groups.
Unfortunately, most communication outlets are likely to be spammed to some degree. The more valuable the communication medium, the more likely it will attract spammers. Ironically, spammers threaten the very mediums they profit from.
see also: Digital Marketing Tools