The term indulgence in Roman Catholicism refers to the plenary (full) or partial remission of temporal punishment due for sins whose guilt and eternal punishment have already been pardoned. Indulgences are usually granted in exchange for prayers and devotional acts.
During the medieval period, this practice came under abuse when indulgences could be obtained through monetary contributions, one factor which eventually instigated the Protestant Reformation.
Pearls of Wisdom, vol. 28, no. 9.