Short answers, pulled from the story.
Alternate history describes a subgenre where one or more historical events unfold differently than in real life. This genre requires three conditions: a point of divergence from actual history, significant alteration of known history, and examination of the ramifications on the world.
Louis Geoffroy released Histoire de la Monarchie universelle between the 18th of December 1812 and the 31st of December 1832. The work imagines Napoleon conquering England in the year 1814.
Harry Turtledove became known as Master of Alternate History with his Timeline 191 series and Worldwar series. These works define his reputation within the genre alongside other major authors like Robert A. Heinlein and Ray Bradbury.
Philip K. Dick published The Man in the High Castle in 1962 depicting Nazi Germany and Imperial Japan winning World War II. The story explores an alternative timeline where Axis powers quickly win and install puppet governments.
Disney's Pirates of the Caribbean films take place roughly late 1720s or mid-1750s while altering events like Blackbeard death at Ocracoke Inlet in 1718. The franchise maintains historical authenticity while changing specific dates and outcomes from real history.