Short answers, pulled from the story.
Alternate history is a subgenre of speculative fiction in which one or more historical events occur differently from how they did in reality. Stories in the genre typically require three elements: a point of divergence from the historical record, a change that alters subsequent events, and an exploration of the consequences of that divergence.
One of the earliest known examples appears in the Roman historian Livy's Ab Urbe Condita Libri, in Book IX, sections 17-19, where he contemplated an alternative 4th century BC in which Alexander the Great survived to invade Europe and concluded that Rome would likely have defeated him. An even earlier instance of speculative material appears in Herodotus's Histories.
The first known complete alternate history in English is Nathaniel Hawthorne's short story "P.'s Correspondence", published in 1845. It depicts a man considered mad for perceiving a different 1845 in which long-dead figures including Robert Burns, Lord Byron, Percy Bysshe Shelley, John Keats, Edmund Kean, George Canning, and Napoleon are still alive.
The Sidewise Award for Alternate History is named after Murray Leinster's 1934 story "Sidewise in Time", which has been described as the point at which alternate history first entered science fiction as a plot device. In the story, pieces of Earth swap places with their counterparts from different timelines.
Uchronia is an English loanword derived from equivalents in Spanish, French, German, Portuguese, Italian, Catalan, and Galician. In English it began as a synonym for alternate history but now also refers to a broader umbrella genre encompassing parallel universes in fiction and fiction set in futuristic or non-temporal settings.
AlternateHistory.com describes itself as the largest gathering of alternate history fans on the internet, with over ten thousand active members. The dedicated Usenet newsgroup soc.history.what-if was created in May 1995 and preceded the forum, itself preceded by the Usenet Alternate History List first posted on the 11th of April 1991.