Any bowed string musical instrument may be informally called a fiddle, regardless of the kind of music being played with it. Violins or other members of the violin family are often affectionately referred to by their players as "my fiddle".
The bowed string instrument first appeared in India circa 3000 BC, and is described in Hindu myth as Ravanahatha. From India, the technology traveled out both to China, and through Central Asia to Europe.
The medieval fiddle emerged in 10th-century Europe, deriving from the Byzantine lira (Greek:λύρα, Latin:lira, English:lyre), a bowed string instrument of the Byzantine Empire and ancestor of most European bowed instruments. The first recorded reference to the bowed lira was in the 9th century by the Persian geographer Ibn Khurradadhbih (d. 911); in his lexicographical discussion of instruments he cited the lira (lūrā) as a typical instrument of the Byzantines and equivalent to the rabāb played in the Islamic Empires. Lira spread widely westward to Europe; in the 11th and 12th centuries European writers use the terms fiddle and lira interchangeably when referring to bowed instruments (Encyclopædia Britannica. 2009).
Over the centuries, Europe continued to have two distinct types of fiddles: one, relatively square-shaped, held in the arms, became known as the lira da braccio (arm viol) family and evolved into the violin; the other, with sloping shoulders and held between the knees, was the lira da gamba (leg viol) group. During the Renaissance the gambas were important and elegant instruments; they eventually lost ground to the louder (and originally less aristocratic) lira da braccio family.
Fiddle has a more generalized meaning than violin. Whereas violin refers to a specific instrument, fiddle may be used to refer to a violin or any member of a general category of similar stringed instruments played with a horsehair bow, such as the Hardanger fiddle, the Byzantine lira, the Chinese erhu, the Welsh crwth, the Apache Tzii'edo' a 'tl, the cello in the context of a Scottish violin/cello duo ("wee fiddle and big fiddle"), the double bass ("bull fiddle" or "bass fiddle"), and so on.
The etymology of fiddle is uncertain: the Germanic fiddle may derive from the same early Romance word as does violin, or it may be natively Germanic. The name seems however to be related to Icelandic Fiðla. A native Germanic ancestor of fiddle may even be the ancestor of the early Romance form of violin. Historically, fiddle also referred to a predecessor of today's violin. Like the violin, it tended to have four strings, but came in a variety of shapes and sizes. Another family of instruments which contributed to the development of the modern fiddle are the viols, which are held between the legs and played vertically, and have fretted fingerboards.
Common distinctions between violins and fiddles reflect the differences in the instruments used to play classical and folk music. However, it is not uncommon for classically trained violinists to play folk music, and today many fiddle players have some classical training. A lot of traditional (folk) styles are oral traditions, so are taught 'by ear' rather than with written music.
In construction, fiddles and violins are essentially identical (with the Norwegian Hardanger fiddle excepted as a special case). The medieval fiddle had rear tuning pegs set in a flat headstock similarly to the Byzantine lyra and unlike the rabāb and rebec.
Some (folk) fiddle traditions fit the instrument with a flatter bridge than classical violinists use. The difference between "round" and "flat" is not more than about a quarter or half a millimeter variation in the height of one or two strings, but is sufficient to reduce the range of right-arm motion required for the rapid string-crossings found in some styles, and those who use flatter bridges say it makes playing double stops and shuffles (bariolage) easier. It can also make triple stops possible, allowing one to play chords. In bluegrass and old-time music, for example, the top of the bridge is sometimes cut so that it is very slightly flattened; the Hardanger fiddle uses an even flatter bridge, and the bridge of the kontra or bracsa (a three-string viola used in Hungarian and Transylvanian folk music) is flat enough that all three strings can easily be played simultaneously.
Most classical violinists prefer a more rounded curve to the top of the bridge, feeling that this allows them to articulate each note more easily and clearly. Many fiddle players use the same top curve as well; most fiddles are fitted with a standard classical bridge, regardless of the style of music played on the instrument. Since the bridge may be changed, it does not permanently define an instrument as fiddle or violin.
Fiddle is more likely to be used than violin if the instrument's strings are steel rather than gut or synthetic, as the sound of steel strings better suits some fiddling styles. Tuning steel strings is easier with fine tuners (small screw mechanisms attached or built into the tailpiece) so fiddlers may favor instruments with fine tuners on all four strings; it is very uncommon to see four fine tuners on full-size instruments played by classical musicians. Strings are subject to regular replacement, fine tuners may be added or removed, and tailpieces may be changed, so, like flattened bridges, they do not make an irreversible difference.
Various clichés describe the difference between fiddle and violin: "When you are buying it, it's a fiddle. When you are selling it, it's a violin." "What's the difference between a violin and a fiddle? About $10,000." "The difference is in the nut that holds the bow." "The violin sings, the fiddle dances." "A fiddle is a violin with attitude." "No one cries when they spill beer on a fiddle." "The difference between a violinist and a fiddle player is $100 a night, and a tux." "A violin is carried in a violin case, a fiddle is carried in a gunnysack." According to the performer Shoji Tabuchi, the difference lies "in how you fiddle around with it."