A viol is any member of a family of six-stringed musical instruments, with a fretted finger board, with shoulders sloping up to the neck unlike the violin family, a flat back, and “C” shaped sound holes. It was a popular solo and ensemble instrument in European music from its appearance (probably from Africa) during the 15th century to well into the 18th century.
They range in size from high treble, alto, small tenor, tenor, bass down to the contra-bass (the "violone") Unlike the violin family – indeed most instrumental “families” the leading viol is a low one. The bass-viol or “viola da gamba", literally “leg-viol”, has had a distinguished career as soloist and is the only member to be honoured by solo works by as high a luminary as J.S. Bach. In England a consort of viols was used either as an instrumental ensemble on its own or as accompaniment in a vocal work. The viola da gamba has enjoyed a resurgence in modern times, not least because of the scholarship and virtuosity of the Catalonian viol player, Jordi Savall.
“Oxford Companion to Music”
“The Grove Concise Dictionary of Music”