No. 1471, page 370 of the Catechism of the Catholic Church: The Celebration of the Christian Mystery; Chapter Two - The Sacraments of Healing; Libreria Editrice Vaticana: Urbi Et Orbi Communications:
"An indulgence is a remission before God of the temporal punishment due to sins whose guilt has already been forgiven, which the faithful Christian who is duly disposed gains under certain prescribed conditions through the action of the Church which, as the minister of redemption, dispenses and applies with authority the treasury of the satisfaction of Christ and the saints." (Paul VI, apostolic constitution, Indulgentiarum doctrina, Norm 1.)
"An indulgence is partial or plenary according as it removes either part or all of the temporal punishment due to sin." (Indulgentiarum doctrina, Norm 2; cf. Norm 3) Indulgences may be applied to the living or the dead.
From the Baltimore Catechism No. 3; Benziger Brothers, Inc. and Tan Books and Publishers, Inc. Nos. 839 through 841, pages 177-178.
Q. What is an indulgence?
A. An Indulgence is the remission in whole or in part of the temporal punishment due to sin.
Q. What does the word "indulgence" mean?
A.The word "indulgence" means a favor or concession. An indulgence obtains by a very slight penance the remission of penalties that would otherwise be severe.
Q. Is an indulgence a pardon of sin, or a license to commit sin?
A.An indulgence is not a pardon of sin, nor a license to commit sin, and one who is in a state of mortal sin cannot gain an indulgence.