Da capo meaning to return 'to the head'. By about 1650 the binary aria form was being extended by a repetition of the A section after the B section was completed. This created the musically satisfying (though dramatically problematic) form, A B A'. In the eighteenth century such arias would become a superb vehicle for vocal display.
a 3-part aria, in ABA form. The first A section is contained within the tonic key, and ends with a clear cadence. The B section is contrasting in character and sometimes also in key. A complete repeat of A follows. It was common practice for the vocal soloist to improvise ornamentation on the repetition of A. Common from the second half of the 17th century through about 1770, especially in the works of Alessandro Scarlatti, Handel, Porpora, Vinci, and, yes, Bach.