though the term implies three (trio), a trio sonata normally employs 4 performers--two melodic parts, and two instruments (a melodic one and a chordal one) on the continuo part. There are, however, three separate parts in the score.

Baroque chamber sonata type written in three parts: two melody lines and the basso continuo; requires a total of four players to perform.

A type of Baroque chamber music written in three parts (two upper voices plus a thoroughbass part). A trio sonata is normally performed by four instruments, e.g., two violins in the upper voices, a violoncello or bassoon to play the bass line, and an instrument such as a harpsichord or lute to provide the realization of the thoroughbass accompaniment. For cataloging and classification purposes, a trio sonata is considered a trio. (Thorin)

A baroque sonata for three main instruments plus the continuo chord instruments.

Sonata for two main instruments, always accompanied by the continuo.

The trio sonata is a musical form which was particularly popular around the 17th century and the 18th century.