A Picardy Third (or Tierce de Picardie) is where a major chord is written as the final chord of a piece that has mostly been in the minor key. This is achieved very simply by raising the minor 3rd of the expected minor chord by a semitone to create a major 3rd.

What is the opposite of a Picardy Third?

Reverse Picardy Third

The "reverse" Picardy third, where an expected major chord is replaced by its minor equivalent, is almost never used at the end of a work - an example of this rarity is in Mendelssohn's Characteristic Piece Op.

How do you do a 3rd Picardy?

It is achieved by raising the third of the expected minor triad by a semitone. For example, in a piece in A minor, instead of ending on the minor triad chord of A C natural E, introducing a Picardy Third would raise the C natural by a semitone to C sharp, and turn the chord into an A major triad.