Originally from "banjar," an African string instrument. Some etymologists derive it from a dialectal pronunciation of "bandore".
The banjo comes in a variety of different forms, including four-string (or plectrum) and five-string versions. In all of its forms it is a poorly sustaining instrument and its playing is characterised by a fast strumming or arpeggiated right hand, although there are in fact many different playing styles.
The banjo consists of a wooden or metal drum, used as a soundboard and often with a ring made of metal, a neck mounted on the side of the drum, a tailpiece mounted on the opposite side, four or five strings, and a bridge. In the five-string banjo, the fifth peg is normally on the side of the neck, but it may be on the tuning head with the others, and the string pass through a tube. Some banjos have a resonator on the back of the drum or a wristpiece on the edge of the drumhead. The drumhead was traditionally made of vellum, although plastic is now a very commonly used substitute. The banjo neck is usually fretted, although fretless versions do also exist. The strings are most commonly metal, but nylon is often used and in the past gut was common.
The banjo can be played in several styles and is used in various forms of music. In bluegrass music, which uses the five-string banjo extensively, it is often played in Scruggs style. American old-time music also typically uses the 5-string banjo, but it is played in different styles, notably claw-hammer or frailing. Another characteristic of old-time banjo styles is the use of a wide range of different tunings.
Many tunings are used for the five-string banjo. Probably the most common, certainly in bluegrass, is the open G tuning: gDGBd. In earlier times, the tuning gCGBd was commonly used instead. Other tunings common in Old-time music include double C (gCGCd), sawmill (gDGCd), and open D (f#DF#Ad). These tunings are often taken up a tone, either by tuning up or using a capo.
The fifth string is the same gauge as the first, but it is five frets shorter (3/4 as long). This presents special problems for using a capo to change the pitch of the instrument. For small changes (e.g. going up or down a (semi)tone) it is possible to simply retune the fifth string. Otherwise various devices are available for effectively shortening the string. Many banjo players favour the use of model railroad spikes (usually installed at the 7th fret and sometimes at others), under which the string can be hooked to keep it pressed down on the fret.
The plectrum banjo has four strings and is missing the shorter fifth string; it is usually tuned DGBd. As the name suggests, it is usually played with a plectrum unlike the five-string banjo which is almost always played with fingerpicks or bare fingers. The plectrum banjo evolved out of the 5-string banjo to cater for styles of music involving strummed chords. A further development is the tenor banjo, which also has four strings and is typically played with a plectrum. It has a shorter neck than the other banjos and is usually tuned CGDA, like a viola, or GDAE, like a violin (but an octave lower), and has become quite a standard instrument for Irish traditional music.
A number of hybrid instruments exist, crossing the banjo with other stringed instruments. Most of these use the body of a banjo, often with a resonator, and the neck of the other instrument. Examples include the guitar banjo and the ukulele banjo. These were especially popular in the early decades of the twentieth century and were probably a result of a desire either to allow players of other instruments to jump on the banjo bandwagon at the height of its popularity or to get the natural amplification benefits of the banjo resonator in an age before electric amplification. Instruments using the five-string banjo neck on a wooden body (for example, that of a bouzouki) have also been made, though these are not so common.