SAO, or Sword Art Online, is a popular Japanese light novel series written by Reki Kawahara. It follows the story of players trapped in a virtual reality MMORPG where dying in the game means dying in real life. The protagonist, Kirito, must navigate this dangerous world to find a way to escape. The series has expanded into various media, including anime, manga, and video games. It explores themes of friendship, survival, and the impact of technology on society. SAO has gained a large fanbase and has influenced the genre of virtual reality storytelling in anime and gaming.