架空历史小说(Alternate history、alternative reality)，简称架空小说，即描述「并非真实发生的虚构历史」的小说，包括历史背景及未来。小说的最大不同在于故事通常会发生在一个作者以某些创作前提（如娱乐性、特定背景等）为本所虚构的或被改编的历史，一个由作者随意设定的世界。

《康州美国佬在亚瑟王朝》是美国作家马克·吐温的一部著名的长篇小说，这部小说开创了一百年后风行的“穿越小说”这一类型，被誉为“穿越文鼻祖”。

Alternate history or alternative history, sometimes abbreviated as AH, is a genre of fiction consisting of stories in which one or more historical events occur differently from reality. These stories usually contain "what if" scenarios at crucial points in history and present an outcome of events alternative to historical record. The stories are the product of conjecture, but are sometimes based on scientific fact. Alternate history can be seen as a subgenre of literary fiction, science fiction, and/or historical fiction; different alternate history works may use tropes from any or all of these genres. Another term occasionally used for the genre is "allohistory" (literally "other history").