Answer:
One that is set in a historical period so accurately that the flaws in it are difficult to identify and refute. But it is not just a convincing period piece. It includes historical figures and events within the story itself. Historical novels often have the feel that the author is giving us the 'back story' that we always wanted to know. Examples are Aztec by Gary Jennings, and The Journeyer, by the same author, about the travels of Marco Polo. You could include novels like Ancient Evenings by Normal Mailer, about mystical events in ancient Egypt.