What does the word "Ham" mean, in the context of anti-spam?

Nowadays, it's likely that everyone knows what Spam means, in the context of e-mail. The use of the word "Ham", on the other hand, is relatively new and sometimes confusing.

"Ham" is e-mail that is not Spam. In other words, "non-spam", or "good mail". It should be considered a shorter, snappier synonym for "non-spam".

Its usage is particularly common among anti-spam software developers, and not widely known elsewhere; in general it is probably better to use the term "non-spam", instead.

Etymology

The word was coined sometime in 2001 or 2002, by the guys working on SpamBayes, the Python probabilistic classifier.