The light novel series spans several virtual-reality worlds, not exclusively the eponymous world of Sword Art Online.

Sword Art Online (SAO) (ソードアート・オンライン Sōdo Āto Onrain^?)

The first virtual game world, and the setting of the first story arc. The world takes the form of a giant floating castle called Aincrad, with 100 floors in it. Each floor has a medieval-themed setting and a dungeon with a boss, which has to be defeated before players can advance to the next higher floor. Like most RPGs, it implements a level-based system. However, after the beta testing, the game's creator activated a system to trap the players inside the game, unable to log out; if players die in-game, or the helmets they used are removed, a strong, electromagnetic pulse is emitted, frying their brain and killing them.