Zhè bù xiǎoshuō shì yī gè jiàkōng de lìshǐ shìjiè.
This novel is set in an alternate-history world.
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架空 has three distinct registers: (1) literal — overhead/elevated structures; (2) political — to neutralize someone's authority while leaving the title; (3) literary — 架空历史 (alternate history) is a major genre in Chinese web fiction.