Well, yes and no, it depends. What it depends on is who's playing it, and in what cultural context you're speaking.

Supposing that one is from the Midwestern United States, “fiddle” may refer to country and western fiddle.

However, you hear violinists of the highest caliber, like Stern and Perlman, for example, refer to the violin as a fiddle...but their cultural context is Eastern European, which included gypsy-like so-called "fiddle" music, which is not the same at all as the American genre.

The instrument itself may be the same, though folk players of violin (and other players who are playing something besides art music) may take more liberties with respect to the way the instrument is held, its fittings, and so on.

Aside from some small details, however, the instrument is pretty much the same; there is no separate genre, fiddle, which is not also a violin. This question gets asked a lot.

Connie Sunday: Freelance studio musician, strings and piano teacher, author of Violin FAQ and numerous essays in violin pedagogy. Further information can be found at connexions.