Metroidvania is a subgenre of action-adventure video games. The term is a portmanteau of the game series Metroid and Castlevania. Metroidvania games use game design and mechanics similar to games from these two series. Specifically, the term derives from the Castlevania title Castlevania: Symphony of the Night and many of the games in the franchise which come after it, which are generally considered containing certain aspects of gameplay comparable to that of the Metroid series of games. The term is used to invoke gameplay concepts and mechanics similar to that of these two series. Koji Igarashi, the assistant director for Castlevania: Symphony of the Night, is credited with establishing key principles of Metroidvanias through his work on other Castlevania games.