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.

Grove Music Online 2019, Tierce de Picardie, Rushton, Julian, viewed 07 July 2019, Published in print: 20 January 2001, Published online 2001.