Ninja Gaiden (NINJA 外伝) is a series of video games by Tecmo featuring the ninja Ryu Hayabusa as its protagonist. The series was originally known as Ninja Ryukenden (忍者龍剣伝 Ninja Ryūkenden, lit. "Legend of the Ninja Dragon Sword") in Japan. The word "gaiden" in the North American Ninja Gaiden title means "side-story" in Japanese, even though the Ninja Gaiden series is not a spinoff of a previous series. The original arcade version, first two NES games and Game Boy game were released as Shadow Warriors in PAL regions.
The series gained popularity on the 8-bit Nintendo Entertainment System (NES). The 8-bit trilogy was enhanced for the 16-bit Super NES in 1995. Sega also released two Ninja Gaiden games for the Game Gear and Master System, the latter only for PAL regions. A new game, titled Ninja Gaiden, was released in 2004 as a 3D action game on the Xbox, developed by Team Ninja, the makers of Dead or Alive. More: en.wikipedia.org.