|00:49The term "Fifth String" almost always refers to the short drone string unique to a five string banjo and American fretted instruments. Lots of instruments have fifth strings, but the banjo's fifth string is tuned and positioned higher than the rest, unlike the fifth string on other instruments, which is usually the fifth highest in pitch.
It may seem news to some, but the five string banjo was the original banjo style. Later, shortly after 1900, the fifth string was removed to form the four string tenor and plectrum banjo styles, which carried the rhythm of early jazz bands. The five string banjo remained the rural musician's favorite for "old-time" country, folk and post-World War II Bluegrass music.