double bass

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. 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.

Century Dictionary and Cyclopedia

1. n. A musical instrument, the largest and deepest of the viol family, having 3 or 4 strings, with a compass of over 3 octaves from the third E below middle C. It was invented in the sixteenth century, and introduced into the orchestra about 1700; and it is now one of the most useful of orchestral instruments. The strings are usually tuned a fourth apart.

Wiktionary

1. n. music The largest stringed instrument of the violin family.
2. n. The deepest of all male voices.

WordNet 3.0

1. n. largest and lowest member of the violin family
2. adj. pitched an octave below normal bass instrumental or vocal range