penance

The deep source is Latin paenitentia, meaning repentance, regret, or inward sorrow for wrongdoing. It belongs to the same family as paenitere, to cause regret or to repent. In Roman usage the word named a moral state before it named an imposed act. The feeling came first.

Christian Latin gave paenitentia a sharper religious role. By late antiquity, it referred both to repentance and to the sacramental discipline attached to repentance. Bishops and canon lawyers turned inward remorse into outward practice. Prayer, fasting, and exclusion could all fall under the term.

Old French developed penitence and penance side by side, and Middle English adopted both. Penance came to stress the imposed or performed act of satisfaction, while penitence stayed closer to the inward condition. English thus split one older idea into two neighboring words. The Church's discipline shaped that division.

Modern usage keeps that history visible. Penance may still refer to a sacrament or an act assigned by a confessor, but it also names any burdensome task accepted as reparation. What began as remorse became a deed. An inward ache became an outward payment.