Look up the word “curveball” in the official glossary located on MLB’s homepage and you will see this as a definition:
A curveball is a breaking pitch that has more movement than just about any other pitch. It is thrown slower and with more overall break than a slider, and it is used to keep hitters off-balance. When executed correctly by a pitcher, a batter expecting a fastball will swing too early and over the top of the curveball.
Hmm, nothing there that gives an actual definition of what a curveball is, but read a bit further and this pops up [bolding mine]:
A curveball can be thrown with a number of different grips.