Tierce de picardie

A Picardy third (also known as Tierce de Picardie) is a harmonic device used in European classical music.It refers to the use of a major chord of the tonic at the end of a musical section in a minor key. This is achieved by raising the third of the expected minor triad by a semitone (a half step), to create a major triad. The aural effect of this is unexpected, and can add a "bittersweet" feeling to the music.
See more at Wikipedia.org...