.hack comprises "Project .hack" and ".hack Conglomerate". It is a Japanese multimedia franchise primarily developed by CyberConnect2 and published by Bandai. The franchise is set on an Earth with an alternate history. In this timeline, a new version of the Internet arises following a major global computer network disaster in 2005. Central to the premise is a mystery about the wildly popular in-universe video game, The World. As most of the story takes place within The World, characters typically interact as their avatars.

The first four video games follow a newcomer called Kite, who goes on a quest to revive his friend, who fell into a coma after his character was killed by an unknown creature. Bee Train also produced an anime series focused on a player named Tsukasa, who has no memory of his identity outside of the game and cannot leave The World. In 2006, Cyber Connect 2 and Bee Train produced two new series, .hack//G.U. and .hack//Roots. The two series involve a returning player by the name of Haseo who, after the loss of his guild, goes on a quest to find the player killer (PK) Tri-Edge who sent his friend Shino into a coma. The latest game by CyberConnect2 is .hack//Link, where Tokio Kuryuu is transported into The World: RX and time-travels across the previous storylines to restore frozen players.

.hack was conceived by CyberConnect2 with the idea of creating a fictional MMORPG in order to simulate a realistic story. The initial characters were designed by Yoshiyuki Sadamoto, while later installments involved additional artists. The critical reception to the original characters has been positive for their designs and realistic traits. The G.U. characters received mixed responses due to Hero's anti-heroic characterization but praise for his growth.