A means of repairing a sin committed, and obtaining pardon for it, consisting partly in the performance of expiatory rites, partly in voluntary submission to a punishment corresponding to the transgression. Penance is the fourth of seven sacraments in the Roman Catholic Church.
This is me, a sinner on whom the Lord has turned his gaze. And this is what I said when they asked me if I would accept my election as pontiff. I am a sinner, but I trust in the infinite mercy and patience of our Lord Jesus Christ, and I accept in a spirit of penance. Pope Francis
Three conditions are necessary for Penance: contrition, which is sorrow for sin, together with a purpose of amendment; confession of sins without any omission; and satisfaction by means of good works. Thomas Aquinas