The nature or essence of a thing; the internal specific principle of the distinctive nature or activities of any created being. Also, after a change, the new feature(s) in whatever has been changed. Contrasted with matter in scholastic philosophy and theology.
In reference to the sacraments, their form is the sacramental sign or the words that specify the function of the matter and confer on it the power of sanctifying. Thus in baptism the Trinitarian formula "I baptize you . . ." is the form.