A stringed instrument with a narrow fretted neck and a thin membrane or diaphragm of plastic or vellum stretched over a frame or cavity to form a resonator and upon which the bridge rests. The body and membrane are typically circular. Early forms of the instrument were originally fashioned by African Americans in the United States during the slave tarde, adapted from African instruments of similar design such as the bania.

Early, African-influenced banjos were homemade and built around a gourd body and a wooden stick neck. These instruments had varying numbers of strings, although often including some form of drone. The earliest known picture (c. 1785–1795) of a slave playing a banjo-like instrument shows a 4-string instrument with its 4th (thumb) string shorter than the others.

In the antebellum South, many black slaves played the banjo and taught their masters how to play. As professional performances of the banjo began around the 1830s, banjos began to have a more modern style and construction, notably including the addition of a 5th string to the generally 4-string African-American banjo. Banjo innovation which began in the minstrel age continued through to the early 20th century, with increased use of metal parts, exotic wood, raised metal frets and a tone-ring that improved the sound.

By the early 1900s, changing musical tastes towards dance music such as ragtime and the need for louder instruments saw the emergence of new 4-string and tenor banjos alongside the more traditional 5-string models current since the 1830s. These had steel strings and were played with a plectrum rather than with the minstrel-banjo clawhammer stroke or the classic banjo fingerpicking style.

Despite a decline following the Great Depression, the banjo has made a comback in the post-war years, in both 5-string and 4-string, and even 6-string, forms, both a rhythm and a solo instrument in Dixieland jazz, folk and country music. The tenor banjo has become an intrinsic part of the world of Irish traditional music.