[from Monty Python's Flying Circus]
1. To crash a program by overrunning a fixed-size buffer with excessively large input data. See also buffer overflow, overrun screw, smash the stack.
2. To cause a newsgroup to be flooded with irrelevant or inappropriate messages. You can spam a newsgroup with as little as one well- (or ill-) planned message (e.g. asking “What do you think of abortion?” on soc.women). This is often done with cross-posting (e.g. any message which is cross-posted to alt.rush-limbaugh and alt.politics.homosexuality will almost inevitably spam both groups). This overlaps with troll behavior; the latter more specific term has become more common.
3. To send many identical or nearly-identical messages separately to a large number of Usenet newsgroups. This is more specifically called ECP, Excessive Cross-Posting. This is one sure way to infuriate nearly everyone on the Net. See also velveeta and jello.
4. To bombard a newsgroup with multiple copies of a message. This is more specifically called EMP, Excessive Multi-Posting.
5. To mass-mail unrequested identical or nearly-identical email messages, particularly those containing advertising. Especially used when the mail addresses have been culled from network traffic or databases without the consent of the recipients. Synonyms include UCE, UBE. As a noun, ‘spam’ refers to the messages so sent.
6. Any large, annoying, quantity of output. For instance, someone on IRC who walks away from their screen and comes back to find 200 lines of text might say “Oh no, spam”.
The later definitions have become much more prevalent as the Internet has opened up to non-techies, and to most people senses 3 4 and 5 are now primary. All three behaviors are considered abuse of the net, and are almost universally grounds for termination of the originator's email account or network connection. In these senses the term ‘spam’ has gone mainstream, though without its original sense or folkloric freight — there is apparently a widespread myth among lusers that “spamming” is what happens when you dump cans of Spam into a revolving fan. Hormel, the makers of Spam, have published a surprisingly enlightened position statement on the Internet usage.