Another term for a double bass, acoustic bass, or bass fiddle. It's the largest of the string instruments (resembling a giant violin) and lowest-sounding instrument found in an orchestra. Can be played with a bow or plucked with the fingers. The word "upright" means that it is to be played while standing up while holding the instrument upright, as opposed to how a bass guitar is held (on the lap or at a 45 degree angle). Also not to be confused with the cello, which is smaller and played while sitting down.
Paul Chambers and Ron Carter each played the upright bass at one time or another for Miles Davis.