From Isidore's Etymologies, Book VI, 71-79 (my very rough translation):
Penitence (poenitentia) is named as if it were punishment (punitentia), because by repenting in himself a man punishes the bad he has done. For nothing other is done by those who truly repent than that they do not allow the bad they have done to be unpunished. The one whose high and just judgment is not evaded by the contemptuous is sparing to the extent one does not spare oneself. And that penitence is complete that weeps for for past [sins] and does not allow future ones. This is according to the likeness of a fountain, because if when the devil attacks some sin by chance should creep in, by this satisfaction it is purified. And satisfaction is to exclude the causes and temptations (suggestiones) of sins and not to repeat the sin again. But reconciliation (reconciliatio) is what is added after the completion of penitence. For as we are united (conciliamur) to God when first we converted from paganism (a gentilitate), so we are reunited (reconciliamur) we return repentantly (poenitendo) after sin. Exomologesis is the Greek word which is understood in Latin as 'confession', which word has a twofold signification. Either confession is understood as in praise, such as: 'I will confess you, O Lord, the Father of heaven and earth', or as when one confesses his sins so that they will be regarded kindly by the one whose compassion is indeficient. Therefore by use of this Greek word 'exomologesis' we express that act by which we confess our failing to the Lord, not as if the one to whose knowledge nothing is hidden were ignorant, but confession is a declared awareness (professa cognitio)of a thing, namely of that which has been unknown (ignoratur). For suppose a person judged it useful and pleasant to rape, to commit adultery, to steal; but when he becomes aware (cognovit) that these are punishable (obnoxia) by eternal damnation, he confesses his error as he becomes aware (cognitis) of this. And confession is profession (professio) of ceasing to err; therefore there should be a ceasing of sin when there is confession. And confession precedes, remission follows. But beyond pardon is one who is aware (cognoscit) of his sin and does not confess what is known (cognitum). Thus exomologesis is the discipline of prostrating and humbling oneself, in dress and food, lying in sackcloth and ashes, darkening his body with dirt, casting down his spirit in lamentation, changing by harsh treatment those things in which one sins.