Unsolicited bulk email sent out randomly to consumers who have not given the sender permission to communicate with them via email. Incidentally, the generally accepted source of the term is that it comes from the Monty Python sing, “Spam spam spam spam spam spam spam spam, lovely spam, wonderful spam…” Like the song, spam is an endless repetition of worthless text.
Some say however, that it comes from the computer group lab at the University of Southern California who gave it the name because it has many of the same characteristics as the lunchmeat: No one wants it or asks for it, no one ever eats it but sometimes its actually tasty, like 1% of junk mail that is really useful to some people.