An electric violin is simply a violin with an electronic signal output. The term can refer to an acoustic violin with an electric pickup of some type, but usually refers to a dedicated electronic instrument.
Many instruments have a solid body, which helps to prevent feedback from the resonances of the hollow body under high amplification on stage. The timbre of an acoustic violin is created directly because of these resonances, however, so an electric violin has a much more "raw", "sharp" sound than an acoustic, depending on how the signal is picked up. They typically have a non-traditional, minimalistic design to keep weight down.
They are often seen as "experimental" instruments, being less established than electric guitar or bass. Hence, there are many variations on the standard design, such as frets, extra strings, machine heads, "baritone" strings that sound an octave lower than normal, sympathetic strings, and more, without even going into the many electronic effects used to shape the raw sound to suit the player's preference.
It is much more common for an electric violin to have 5 or 6 strings than an acoustic instrument. The typically solid body also accommodates the tension caused by extra strings without stressing the instrument too much. Extra strings are usually a low C string for 5 strings, and a low C and high B for 6, though this is also highly variable.
Electric violin signals usually pass through electronic processing, in the same way as an electric guitar, to achieve a desired sound. This could include delay, reverb, chorus, distortion, or other effects.
Since it (usually) has metal strings, the sound of the violin can be sensed with either magnetic or piezoelectric pickups, but piezo pickups are more common. They are used to detect physical vibrations, sometimes in the body, but more commonly in the bridge. Some setups have a separate pickup (or two!) embedded in the bridge under each string.
Piezo pickups have a very high (capacitive) output impedance, and require a powered preamp (a charge amplifier is best) to avoid signal loss, filtering, and excessive noise pickup in the instrument cable. A solid body provides room for this circuitry and a battery, although phantom power can make the battery unnecessary.
- Bowed Electricity - Electric violin players, makers, equipment, and other resources.