Did anyone else just take this for granted? Apparently ‘gaiden’ is a descriptor word in Japanese, referring to an ‘outside story,’ or a story that happens outside the main continuity. Check the Wikipedia definition of “gaiden.” I always assumed “Ninja Gaiden” was just the name of the old video game. Or possibly the name of the ninja in question.

…Apparently it had further significance.

UPDATE: Wikipedia also provides further background on how Ninja Gaiden’s title was selected. There’s an odd sort of poetry to it.