A secret history (or shadow history) is a revisionist interpretation of either fictional or real history which is claimed to have been deliberately suppressed, forgotten, or ignored by established scholars. "Secret history" is also used to describe an alternative interpretation of documented facts which portrays a drastically different motivation or history from established historical events.
Within the context of fiction, secret history is sometimes used in a science fiction or fantasy setting to preserve continuity with the present by reconciling paranormal, anachronistic, or otherwise notable but unrecorded events with what actually happened in known history. In non-speculative fiction, secret history is used to depict historical events that unfold differently than in the standard historical record; the conceit of these works is that recorded history is wrong, and the story is the secret truth of what really happened. Popular subjects of this genre include Napoleon I of France and Adolf Hitler. In a number of popular works, these historical figures fake their publicized deaths and live on under a low profile, dying in obscurity long after their official deaths (1821 for Napoleon, 1945 for Hitler). By contrast, some works have Hitler killed before 1945, and the rest of his biography lived by an impersonator.
Harry Turtledove has declared that he has little interest in secret history, but is not averse to dabbling in it on occasion. He has specifically identified "Under St. Peter's" as a secret history work. Some other Turtledove stories can be considered secret history, a notable example being "Gentlemen of the Shade." Incidentally, both "Gentlemen" and "Under" are also both firmly in the fantasy genre; namely, they are vampire stories in which vampiric powers are used to accomplish the secret historical events depicted.