"Historical fantasy" is fantasy set in a version of historical Earth where either (A) magic is real but hidden (like an urban fantasy-style masquerade) or declining to the point that it will be essentially gone in the modern day or (B) the world has been changed by the overt existence of magic but still basically resembles historical Earth (like, say, a setting where magic replaces technology and World War 2 is a clash between Allied dragonriders and Axis necromancers, totally ignoring the effects might have had to alter history up to that point), and it nearly always involves making mythical things real and/or borrowing from real-world magical traditions instead of inventing things whole-cloth.
Since Three Hearts and Three Lions involves traveling to a world where the legends of Charlemagne and Ogier the Dane are real and the plot revolves around a war a clash between an alternate Holy Roman Empire and alternate Saracens, it's pretty solidly an example of the second version of historic fantasy. You can see the influence of historical fantasy on early D&D in the fact that all the human nations in Mystara and most of those in Blackmoor are basically fantasy-ified versions of different real-world nations and cultures, in contrast to early Greyhawk where there are certainly real-world influences but most of the nations and cultures there are X-meets-Y blends of real-world cultures rather than straight-up ports and there are plenty of nations without any real-world analogs.
The Narnia books aren't historical fantasy, because while the nations of Narnia heavily draw on real-world cultures for influence and there are portals between Earth and Narnia through which cultural influence could flow, it's still a separate world (and a flat one, at that) with its own culture, geography, history, etc. However, if Narnia were merely a parallel Earth using the actual British Isles, Scandinavia, and the Middle East in place of "Narnia" and "Ettinsmoor" and "Calormen" and so forth, and the Dawn Treader's journey East took them to totally-not-India or totally-not-China, then it could potentially be classified as historical fantasy.
The key difference between science fantasy and low fantasy is that the former retains a veneer of scientific or soft-sci-fi versimilitude in its setting elements and explanations for how its fantastical elements work. For instance, the Dying Earth and Athas are both postapocalyptic worlds with red suns and rare magic and such, but the sun dying in The Dying Earth is due to the sun being a very old star reaching the end of its lifespan while the sun dying in Dark Sun is due to the sun being a big ol' ball of light and positive energy that was drained for fuel by various ancient magical rituals.Originally Posted by Jason
Similarly, the Dying Earth and Westeros both have fragmented and mysterious ancient magics and magical beings reemerging after being hidden away for many years or centuries, but magic in The Dying Earth is explained as being a creation of advanced mathematics and physics, many magical artifacts are actually poorly-understood ancient technological devices, and the magical entities are higher-dimensional beings of various sorts, while magic in A Song of Ice and Fire is pure-fantasy bloodlines and prophecies and mysticism and such.
The major science fantasy influence in early D&D was psionics, where psionic potential in (demi)humans arose from random brain mutations and all of the sciences and devotions had much more modern and scientific-sounding names than spells did, and the various blatantly-technological-but-the-natives-think-it's-magic stuff like the "Nucleus of the Spheres" (an ancient crashed starship reactor core) in Blackmoor or the "golems" (maintenance and security robots), "talismans" (security access cards), and "blasting wands" (laser pistols) from the crashed starship in the Expedition to the Barrier Peaks.
I don't think I've noticed anyone conflate the two thus far in the thread. Certainly people have described being imprisoned in the Wall as an eternal punishment, but if "having your mind and soul dissolved by magically-radioactive mold over bazillions of years" isn't considered a punishment, I don't know what is.Originally Posted by Palanan