1. <messaging> (From Hormel's Spiced Ham, via the Monty Python "Spam" song) To post irrelevant or inappropriate messages to one or more Usenet newsgroups, mailing lists, or other messaging system in deliberate or accidental violation of netiquette.
It is possible to spam a newsgroup with one well- (or ill-) planned message, e.g. asking "What do you think of abortion?" on soc.women. This can be done by cross-posting, e.g. any message which is crossposted to alt.rush-limbaugh and alt.politics.homosexuality will almost inevitably spam both groups. (Compare troll and flame bait).
Posting a message to a significant proportion of all newsgroups is a sure way to spam Usenet and become an object of almost universal hatred. Canter and Siegel spammed the net with their Green card post.
If you see an article which you think is a deliberate spam, DO NOT post a follow-up - doing so will only contribute to the general annoyance. Send a polite message to the poster by private e-mail and CC it to "postmaster" at the same address. Bear in mind that the posting's origin might have been forged or the apparent sender's account might have been used by someone else without his permission.
The word was coined as the winning entry in a 1937 competition to choose a name for Hormel Foods Corporation's "spiced meat" (now officially known as "SPAM luncheon meat"). Correspondant Bob White claims the modern use of the term predates Monty Python by at least ten years. He cites an editor for the Dallas Times Herald describing Public Relations as "throwing a can of spam into an electric fan just to see if any of it would stick to the unwary passersby."
See also netiquette.
2. (A narrowing of sense 1, above) To indiscriminately send large amounts of unsolicited e-mail meant to promote a product or service. Spam in this sense is sort of like the electronic equivalent of junk mail sent to "Occupant".
In the 1990s, with the rise in commercial awareness of the net, there are actually scumbags who offer spamming as a "service" to companies wishing to advertise on the net. They do this by mailing to collections of e-mail addresses, Usenet news, or mailing lists. Such practises have caused outrage and aggressive reaction by many net users against the individuals concerned.
3. (Apparently a generalisation of sense 2, above) To abuse any network service or tool by for promotional purposes.
"AltaVista is an index, not a promotional tool. Attempts to fill it with promotional material lower the value of the index for everyone. [...] We will disallow URL submissions from those who spam the index. In extreme cases, we will exclude all their pages from the index." -- Altavista.