When I'm playing Vivaldi, it's a violin. When I'm playing Billy in the Lowground, it's a fiddle.

I have been told by someone who knows a lot more than me, that in the really old days, "fiddle" was a word used to describe just about any instrument played with a bow.

Personally, I use the terms interchangeably. We are talking about an extremely versatile instrument that can produce an infinite range of sounds and expressions, limited only by one's imagination.