According to wikipedia, the fountain of all accurate knowledge, I am responsible for coining the term "retro-clone," at least in the context of pencil & paper RPGS. For as long as I've had a home page for Goblinoid Games I've had a description of what a retro-clone game is, at least as far as I am concerned.
I've seen the term used all over the place now, sometimes used in ways that confuse me. So I want to define a little more closely what I think a retro-clone is, for whatever my ramblings are worth.
A retro-clone, the way I use the term, refers specifically to a game that attempts to emulate as closely as is legally possible the game rules of another game. That's it. If you've ever encountered the various game emulators for NES, SEGA, and other old cartridge console games, then you have an idea what I mean.
However, I see the term used for other games, such as Castles & Crusades and Mazes & Minotaurs. We should draw a line between retro-clones and retro games in general. Mazes & Minotaurs is a retro game, but it clearly is not closely cloning another game.
Castles & Crusades is in an odd category. It clones elements of Basic D&D, AD&D, and D&D 3.5, mixing them all together for a game that can appeal to 3.xers and old-schoolers. So it is definitely a retro game, to one degree or another, but it does not emulate the rules of a specific single game. As a side note, fans of C&C who are critical of retro-clones have no moral high ground. Cloning elements of three games does not make it qualitatively more "moral" compared to games that only clone one game. IMHO cloning is cloning.
Mutant Future is also a special case, which I would call retro but not a "retro-clone." Since it borrows its core rules from Labyrinth Lord, but other elements from other OGC, it does not directly emulate a specific game.
Anyway, I'd classify this post as one of those "oddball rants" I promise in my blog header.