What is a Picardy third?

Answer 1

A picardy third is when, in a major key, the third of a minor chord is raised making it a major chord. Example: in the key of C minor, the c minor chord is spelled C Eb G. When the picardy third is used, the chord becomes C Major or C E G.

Answer 2

A Picardy third consists of a piece of music involving a minor chord progression resolving into a major chord.

For example, this chord progression: i - iv - v - I (Key of G minor: Gm - Cm - Dm - G Maj)