Pastorale refers to something of a pastoral nature in music, whether in form or in mood.
In Baroque music, a pastorale is a movement of a melody in thirds over a drone bass, recalling the Christmas music of pifferari, players of the traditional Italian bagpipe (zampogna) and reed pipe (piffero). Pastorales are generally in 6/8 or 9/8 or 12/8 metre, at a moderate tempo. They resemble a slowed-down version of a tarantella, encompassing many of the same rhythms and melodic phrases.
Common examples include the last movement of Corelli's Christmas Concerto (Op.6, No.8), the third movement of Vivaldi's Spring concerto from The Four Seasons, the Pifa movement of Handel's Messiah, the first movements of Bach's Pastorale (BWV 590) for organ, and the Sinfonia that opens part II of his Christmas Oratorio as an introduction to the angelic announcement to the shepherds. Scarlatti wrote some examples in his keyboard sonatas, and many other composers in the transition between the baroque and classical eras, particularly French, used this technique. Rossini famously included a Pastorale section in his William Tell Overture. The Italian pastorale Tu scendi dalle stelle, sometimes called "Carol of the Bagpipers" (Canzone d'i zampognari), is a widely popular Christmas carol by St. Alfonso Liguori, and Pietro Yon's Gesù bambino is another.