A ukulele is a small guitar with four strings. a musical instrument with four strings, like a small guitar (ukulele, from uku + lele ). Small Hawaiian four-stringed guitar. It developed out of a similar Portuguese instrument introduced to Hawaii by sailors in the 1870s. It became highly popular in the U.S. after World War I, when it was used in jazz and bluegrass ensembles and more widely as an amateur solo instrument