Original: The word penance derives from Old French and Latin poenitentia, both of which derive from the same root meaning repentance, the desire to be forgiven; (in English see contrition).
Simplified: The word penance comes from the Latin poenitentia, the same root as penitence, which in English means repentance, the desire to be forgiven, see contrition; in many languages there is only one single word for it.