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Spam is any unwanted unsolicited message. Spam is usually sent via email, although it is also increasingly showing up on blog comments.
Electronic spam is named after the Hormel Foods SPAM product, a processed meat product. The canned SPAM product is generally regarded as an inferior product. Hormel has come to terms with sharing its name with the unsolicited messages, as long as the term is used uncapitalized.
The use of the term "spam" to refer to unwanted email comes from a Monty Python skit where everything on the menu came with spam. One character stated that he did not want any spam, but the waitress suggested an item with only a little spam. Furthermore, there's a table of Vikings in that skit (don't ask why) and everytime someone says "spam" they start singing "SPAM, SPAM, SPAM, SPAM, wonderful SPAM…" eventually drowning out all other conversation. So the implication is that the spam is unwanted, but you get it anyway. And that the amount and volume of spam is liable to drown out any meaningful message.
When automated systems are used to classify emails, it is often segregated into "spam" (unwanted messages) and "ham" (good messages).