Picardy:

Tierce de Picardie [ Picardy 3rd ] The raised third degree of the tonic chord, when it is used for the ending of a movement or composition in a minor mode in order to give the ending a greater sense of finality. The term was introduced by Rousseau in his Dictionnaire de musique ( 1767 ); its etymology is unknown. It was commonly used in the 16th century and throughout the Baroque era and was regarded by some writers as standard.