When a character, usually a mentor of some sort, in a movie, novel, comic book, or TV show dies and then comes back as a ghost to continue adding to the story, the same way Obi-Wan did in Star Wars IV.
Just about every Stephen King story has someone pulling an Obi-Wan. Nobody ever stays dead for good. Japanese RPGs too.
(noun) Your bankcard at 12:01am on the day you are paid when you have no money.
While out with your friends, you realise you have no more money because it is wednesday and you get paid on thursday. At 12:01am, your money is sometimes transferred to your account, so you must check your obi-wan because it is now your only hope for keeping the night going