Penance is the act of doing a good deed to make up for past wrongs. Shoveling your neighbor's sidewalk all winter could be your penance for not helping rake the leaves that dropped from your tree into his yard during the fall.
Although the noun penance can mean any remorse for past mistakes, or any voluntary action meant to right the wrong, Penance, with a capital P, also specifically refers to a sacrament in the Catholic Church. When a person receives Penance, he or she confesses sins to a priest, and along with a blessing, receives an order to do something, such as say certain prayers. This assignment is also called penance.