Motu proprio (Lat. “by one’s own impulse”) is a canonical ruling (Polity, Ecclesiastical) issued by the pope on his own accord. Along with the apostolic constitution it is an important form of proclamation in papal legislation. A favorable ruling (rescript) has great force when issued in this form (CIC 1983).
R. R. Gaillardetz, Teaching with Authority: A Theology of the Magisterium in the Church (Collegeville, Minn., 1997).