double bass definition by American Heritage Dictionary
The largest bowed stringed instrument in the modern orchestra, also used frequently in jazz ensembles, especially played pizzicato. The double bass, usually considered a member of the violin family, is tuned in fourths and has the sloping shoulders and flat back characteristic of the viols. It has a deep range, going as low as three octaves below middle C. Also called bass fiddle, bass viol, bull fiddle, contrabass, string bass.
Origin: double, tuned an octave lower than (from the fact that a string or pipe that is twice the length of another gies a pitch an octave lower) + bass2.