Fiddle
Folk-music designation of the violin.
What it means
Fiddle is the designation in common use in the English-speaking world for the violin when it is played in folk-music contexts. The instrument itself is the same; the difference lies in manner of playing, repertoire, and form of transmission. Occasionally the bridge is made flatter to ease double stops and drone sounds, and some traditions use deviating tunings. The term is not a pejorative designation but simply names a different practice of music-making.
Musical use
Fiddle traditions exist in many regions, among others in Ireland, Scotland, Scandinavia, and North America; they differ distinctly from one another in rhythm, ornamentation, and ideal of sound. Common to them is that the music is passed on predominantly orally, that it stands in close connection with dance, and that ornaments are not fixed but part of personal shaping. Drone sounds on open strings, emphasized rhythmic figures, and a strong, direct tone with sparing vibrato shape many of these styles. In classical violin practice the reverse holds, which moves the two worlds far apart in sound, although the instrument is the same.
Performance practice
If you come from classical teaching, the main work lies in letting go of habits: less vibrato, more rhythmic drive in the bow, shorter and more accented strokes. Learn melodies by ear instead of from notes — the flexibility of ornamentation arises only in this way. Watch the dance function: the tempo stays stable, and the accents follow the respective dance type. Work with open strings as a tone colour instead of avoiding them. If you try a flatter bridge or another tuning, get advice so that the instrument comes to no harm, and keep a return to the normal setup open.
Bear in mind that in many of these traditions the rhythm is not divided evenly: characteristic displacements belong to the style and cannot be read from notes. Listen a lot and play with experienced musicians instead of learning from transcriptions. Watch the bow: short, accented strokes and a firm contact point carry the dance. Check whether a retuning is usual, and have the instrument set up professionally if necessary. For dance playing the following holds: the tempo stays stable, even when it becomes strenuous.