It is a melodic technique used in a piece or a movement. It is a repetitive passage for orchestra or choir in baroque music. Ritornello often consists of sequences.
The term "ritornello form" refers to the first and last part of a Baroque solo concerto, a concerto grosso, or an aria. There, ritornello is the introductory theme which is played by all (tutti) and returns throughout the movement in different keys. This is the difference between Ritornello and rondo which returns to the same tonality. The ritornello form exists to Bach, Vivaldi, Telemann and Handel very often. Its construction faded with the advent of sonata form, but people's interest was revived in the 20th century.
During the 14th century the couplet at the end of a Madrigal (Madrigal of the 14th century)also was called ritornello. Additionally, some instrumental pieces in early operas of the Baroque period were called "ritornello" .
Fast dance of the 17th century . Most common in ballets of Jean Baptiste Lully.
Willi Apel: Harvard dictionary of music, second edition, eighth printing, ritornello, ritournelle
BWV 207 no 8 ritonello
"Vi Ricorda" from Monteverdi's "Orfeo"