According to Saint Thomas Aquinas the Sacraments may be described in terms of their matter and form. The idea seems to have been first proposed by William of Auxerre. Thus, for example, the matter for Baptism is water; the matter for the Eucharist is bread and wine. The form of a sacrament consists of the words by which the Sacrament is effected. Thus Saint Thomas held that the form of Penance was "I absolve thee."
The Catechism of Trent states it this way: "Every Sacrament consists of two things, matter, which is called the element, and form, which is commonly called the word."
The term "matter" should be taken broadly to mean that which underlies the sacrament in a manner similar that the way in which matter underlies substance. Thus the penance assigned to a sinner is taken to be the matter of Penance.
Secondly the form of a sacrament may take significantly different forms in different churches. Thus, for example, though Saint Thomas held the form of Confirmation to be "I sign thee with the sign of the cross..." the Byzantine Rite uses the form "The sign of the seal of the Holy Spirit."