So What Exactly Is an "Alien Megastructure"?

Last week, I wrote another article about KIC 8462852, aka Tabby's Star, which is behaving very oddly indeed. It shows sharp dips in brightness that last for hours, as well as much longer ones that last for days (or longer) that indicate something is blocking a substantial fraction of the star's light. The most likely explanation is a cloud of dust with comets or asteroids embedded in it, which can cause the star to dim. However, it's been maddeningly difficult to explain all the star's behavior ...