A religion that pops up when formerly isolated ("primitive") cultures are suddenly confronted by materially rich (usually western) cultures. Best documented in the islands of Melanesia. Upon seeing the riches possessed by the newcomers and denied to them, the natives sometimes form cults based on the idea that their Stuff will be coming shortly.
There will be boats or planes coming along soon to give them their share of all the miraculous stuff these newcomers have. They may build air strips for the planes to land on, and mimic other aspects of the western culture's appearance (marching with wooden guns, making mock-up radios, etc.). The cult often incorporates the belief that the newcomers will lose all their stuff, once the gods realize who is more worthy.
The term cargo cult is vague and inexact, and so many anthropologists don't like to use it. But the idea has taken root in the American memespace. It's a really cool idea, good for movies and books, and isn't going to die out as a common term, even if a few anthropologists dispute its worth.