What is "picardy third"

n. (context music English) a major third in the final tonic chord of a minor-key passage, a device mostly used in Baroque, although occasionally in Classical music, and then adopted much later on in neoclassicism.

A Picardy third, Picardy cadence or, in French, tierce picarde is a harmonic device that originated in Western music in the Renaissance era.

It refers to the use of a major chord of the tonic at the end of a musical section that is either modal or in a minor key. This is achieved by raising the third of the expected minor triad by a semitone to create a major triad, as a form of resolution.

For example, instead of a cadence ending on an A minor chord containing the notes A, C, and E, a Picardy third ending would consist of an A major chord containing the notes A, C, and E. Note that the minor third between the A and C of the A minor chord has become a major third in the Picardy third chord.

The Picardy third does not necessarily occur at the end of a section: it can be found at any perfect cadence or plagal cadence where the prevailing key is minor.