The banjolele is a four-stringed musical instrument

Musical instrument

A musical instrument is a device created or adapted for the purpose of making musical sounds. In principle, any object that produces sound can serve as a musical instrument—it is through purpose that the object becomes a musical instrument. The history of musical instruments dates back to the...

 with a small banjo

Banjo

In the 1830s Sweeney became the first white man to play the banjo on stage. His version of the instrument replaced the gourd with a drum-like sound box and included four full-length strings alongside a short fifth-string. There is no proof, however, that Sweeney invented either innovation. This new...

-type body and a fretted ukulele

Ukulele

The ukulele, ; from ; it is a subset of the guitar family of instruments, generally with four nylon or gut strings or four courses of strings....

 neck. "Banjolele," sometimes also spelled "banjelele" or "banjulele" is a generic nickname given to the instrument, which was derived from the "banjulele-banjo", introduced by Alvin D. Keech in 1917.

The instrument achieved its greatest popularity in the 1920s and '30s, and combines the small scale, tuning, and playing style of a ukulele with the construction and distinctive tone of a banjo, hence the name.
Banjo ukuleles parallel banjo construction, on a smaller scale, in terms of overall construction.