As Gregory says [Isidore, Etym. vi, ch. 19], "a sacrament consists in a solemn act, whereby something is so done that we understand it to signify the holiness which it confers." Now it is evident that in Penance something is done so that something holy is signified both on the part of the penitent sinner, and on the part of the priest absolving, because the penitent sinner, by deed and word, shows his heart to have renounced sin, and in like manner the priest, by his deed and word with regard to the penitent, signifies the work of God Who forgives his sins. Therefore it is evident that Penance, as practiced in the Church, is a sacrament.

- St. Thomas Aquinas

The Sacrament of Penance (also known as Confession or Reconciliation) has four elements: contrition, confession, satisfaction and absolution. In this sacrament, we return to God who raises us from the death of mortal sin and enlivens us from the weakness of venial sin.