In his book, The Salvation Controversy, Catholic Answers apologist Jimmy Akin gives this definition on page 25:
An indulgence is the Church’s lessening of the temporal penalties to which we may be subject even though our sins have been forgiven.
A more technical definition of an indulgence and a definition of a plenary indulgence are given in the Glossary of the Catechism of the Catholic Church (CCC):
INDULGENCE: The remission before God of the temporal punishment due to sin whose guilt has already been forgiven. A properly disposed member of the Christian faithful can obtain an indulgence under prescribed conditions through the help of the Church which, as the minister of redemption, dispenses and applies with authority the treasury of the satsifactions of Christ and the saints. An indulgence is partial if it removes part of the temporal punishment due to sin, or plenary if it removes all punishment (CCC 1471).
