Actually, technically, they mean the same thing. The only difference is that one is a misspelling.
The correct spelling is minuscule.
Miniscule makes more sense superficially because it contains the word mini, which means small. But minuscule is derived from the Latin minusculus, whose root word is minus. Technically, that’s the correct spelling.
But.
Miniscule has been around since the late 19th century, so it’s not new. Not really anyhow. Not only is it not new, but it’s also popular. In fact, Oxford Dictionary reports that according to their corpus, “the spelling miniscule now makes up around 52% of the total use of the word”, including edited sources, such as newspapers and other periodicals.
More people use miniscule than used minuscule.
The Oxford Dictionary lists miniscule as a variant spelling under the minuscule entry. Merriam-Webster also lists it as a variant spelling (albeit a disputed one). Cambridge Dictionary, too, lists it as an alternative spelling, as does Collins and Random House Unabridged Dictionary.
And remember, dictionaries don’t decide what words mean; they just record how people use them. And clearly, people are using miniscule.
It’s so widespread and well-established, that it’s probably fruitless to take the prescriptive approach to pedantically insisting on spelling it as minuscule.
As, Oxford points out, this “is a good example of a word whose spelling is changing”. And that’s okay; that’s how English works. We don’t speak the same English that was spoken 400 years ago, and 400 years from now, English speakers will be speaking significantly different from how we do now.
Now, all that being said, don’t get too excited about miniscule. It’s probably still best to stay away from it in formal writing. Or just check with your editor.
