Just What IS a "Quantum"?

Quantum is one of those words that's a godsend if you're a lazy science-fiction author in need of a plot device, or someone trying to scam people into buying yourcrappy, overpriced jewelry. It evokes scientific knowledge and mystery all at once; it lets things be in two places at the same time, or jump to alternate universes.It's a word that practically everyone's heard, but that relatively few people understand properly. Part of the reason for this is because it has multiple meanings in various...