A bass violin, also known as a violone, is a large, stringed instrument played with a bow. This instrument was the predecessor of the modern-day cello. Instrument makers designed bass violins as part of the larger viola da braccio family.
Originally, the term "viola" meant "stringed instrument." A violino, or violin, thus was a "small viola." By contrast, a violone was a "big viola." When people developed the viola da braccio family, when they referred to violones, they thus meant the largest instrument of the string family. Violoncello literally means "small big viola," which indicates that violincello was bigger than violins and violas but not as big as a violone.
First versions of the bass violin emerged in the 16th and 17th centuries. These had three strings, but later, four strings became standard. The general method was to tune the strings in fifths. The body of the instrument was hollow, which provided resonance when players drew the bow across the strings.
The contemporary cello sounds one octave below the viola. Bass violins, by comparison, generally were tuned one pitch lower than this, with the lowest string sounding a B-flat instead of a C. The fact that violincellos developed over many years, however, means that there was some flexibility in terms of tuning, as instrument makers were experimenting with different sizes.
Bass violins are no longer standard in the violin family. Bass violins fell out of favor in part because music became more complex, demanding instruments that were easier to play. The smaller size of the violincello compared to the bass violin permitted greater virtuosity, as the cello was easier to position on the body and allowed freer movement of the fingers. The modern double bass has replaced the bass violin, but because violone technically just means "big viola," some people refer to double basses as bass violins.
Most contemporary performances using a true bass violin focus on period repertoire. Period players generally have extensive knowledge of the differences between the modern cello and the bass violin. This knowledge allows the players to perform the repertoire with a high degree of authenticity.
In particular, a true bass violin tends to have a shorter neck and fingerboard compared to the cello. The neck and fingerboard on a bass violin also isn't as curved. The bridge is lower, as well, and the strings are a bit thicker and not as taut. These differences mean that the sound of a bass violin isn't quite as clear as the sound of a cello, and that the bass violin also isn't quite as responsive.