From Pope St. John Paul II’s Apostolic Exhortation Reconciliatio et Pænitentia (1984):
The term and the very concept of penance are very complex. If we link penance with the metanoia which the synoptic[ Gospel]s refer to, it means the inmost change of heart under the influence of the word of God and in the perspective of the kingdom.9 But penance also means changing one’s life in harmony with the change of heart, and in this sense doing penance is completed by bringing forth fruits worthy of penance:10 It is one’s whole existence that becomes penitential, that is to say, directed toward a continuous striving for what is better. But doing penance is something authentic and effective only if it is translated into deeds and acts of penance. In this sense penance means, in the Christian theological and spiritual vocabulary, asceticism, that is to say, the concrete daily effort of a person, supported by God, to lose his or her own life for Christ as the only means of gaining it;11 an effort to put off the old man and put on the new;12 an effort to overcome in oneself what is of the flesh in order that what is spiritual13 may prevail; a continual effort to rise from the things of here below to the things of above, where Christ is.14 Penance is therefore a conversion that passes from the heart to deeds and then to the Christian’s whole life.
In each of these meanings penance is closely connected with reconciliation, for reconciliation with God, with oneself and with others implies overcoming that radical break which is sin. And this is achieved only through the interior transformation or conversion which bears fruit in a person’s life through acts of penance.
9 Cf Mt 4:17; Mk 1:15.
10 Cf Lk 3:8.
11 Cf Mt 16:24-26; Mk 8:34-36; Lk 9:23-25.
12 Eph 4:23f.
13 Cf 1 Cor 3:1-20.
14 Cf Col 3:1f.
— Reconciliatio et Pænitentia 4