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This is a Japanese light novel series written by Reki Kawahara and illustrated by abec. The series takes place in the near-future and focuses on various virtual reality MMORPG worlds.

The light novels began publication on ASCII Media Works' Dengeki Bunko label from April 10, 2009, with a spin-off series launching in October 2012, and are licensed in North America by Yen Press. The series has spawned five manga adaptations.