In 2005, I attended the first North American conference of the Historical Novel Society, which included the chance to pitch agents and editors. I sent in my query and first five pages and got the following reply back from a well-known agent: she declined to meet with me at the conference because my alternate-history manuscript was "not historical fiction. Historical fiction is what might have happened."
(Rant ahead: that's what my story was! What might have happened if Anne Boleyn had not miscarried her son in January 1536. What might have happened if she'd never had her head cut off and her son became king in 1547. What might have happened if Anne and Henry's son repeated his parents' dangerous forays into love and choice. And now we'll see what will happen when that same story is published as a trilogy by Random House beginning in summer 2013!)
Still, despite my disagreement with that agent's interpretation, I like her actual definition. Historical fiction is what might have happened.
Sometimes it's about the well-known dead; sometimes it's fictional ghosts; often it's a mix. But always it tells a story set in the past, in which the time itself is a critical character. The author recreates buildings, clothing, smells and tastes and sounds that are wildly different from any that can be found today. The story lines reflect religious beliefs and social pressures and power structures that seem nearly as alien as any created in science fiction.
And yet, miraculously, the people are just people. Human beings are fundamentally the same, if not on the outside then always on the inside. Love, hate, fear, hope, anger, desire . . . the best historical fiction does what all best fiction does: shows us ourselves.
This is why I read and write historical fiction: to travel to a time and place I can never get to in real life, and to find myself when I get there.
Recommended reading (those titles that made me say "If this isn't how it happened, it's definitely how it should have"):
HERE BE DRAGONS/Sharon Kay Penman/medieval Wales and England and the royal marriage of Llewellyn the Great and the illegitimate daughter of King John
THE BROTHERS OF GWYNEDD QUARTET/Ellis Peters/also medieval Wales and England, two generations later when England sets its sights on destroying Welsh autonomy
THE LYMOND CHRONICLES/Dorothy Dunnet/six book series about a Scottish soldier-statesman in the Tudor era
THE KING'S GENERAL/Daphne du Maurier/one of my adolescent favorites, set during the English civil war
CLOCKWORK PRINCE & CLOCKWORK ANGEL/Cassandra Clare/a prequel series to Clare's well-known Mortal Instruments books, set in Victorian London
WOLF HALL/Hilary Mantel/last year's Man Booker prize-winner about Thomas Cromwell in the time of Henry VIII, the sequel will be out this year
And for those interested in more titles and/or learning more about historical fiction, check out the Historical Novel Society website. Which, by the way, specifically includes alternate history in its definition of historical fiction. So there.