'Alternate history' - or, as grammatical purists call it, 'alternative history' - is an inaccurate but common label for a small but intriguing body of literature. An alternate history is not a history at all, but a work of fiction in which history as we know it is changed for dramatic and often ironic effect. Often an alternate history dramatizes the moment of divergence from the historical record, as well as the consequences of that divergence. Such a story or novel might seem at first to be a work of traditional historical fiction, in which invented characters and events are woven into the known tapestry of history, but the alteration announces itself quickly, usually in the first few pages.

An example is ‘The Lucky Strike’ by Kim Stanley Robinson (1984), which begins:

For several pages, the fictional adventures of Captain January seem to have a nonfictional backdrop, for the setting meshes with the reader’s knowledge, however vague, of the closing months of the Second World War, as US forces in the Pacific ready atom bombs to drop on Japan. Then comes a spectacular moment of divergence, as the horrified January watches a plane in trouble: