"The remission before God of the temporal punishment due to sins forgiven as far as their guilt is concerned, which the follower of Christ with the proper dispositions and under certain determined conditions acquires through the intervention of the Church, which, as minister of the redemption, authoritatively dispenses and applies the treasury of the satisfaction won by Christ and the saints" (Pope Paul VI, Apostolic Constitution on Indulgences).
As originally understood, an indulgence was a mitigation of the severe canonical penances imposed on the faithful for grave sins. The term "indulgence" remained, however, even after these extreme penalties were discontinued. Yet until the Second Vatican Council, the norm for determining the effectiveness of an indulgenced practice was its relationship to the ancient canonical penances, as seen in the numbers, so many years or so many days, attached to every official listing of partial indulgences.
All this changed by Pope Paul VI. From now on the measure of how efficacious an indulgenced work is depends on two things: The supernatural charity with which the indulgenced task is done, and the perfection of the task itself.
Another innovation is that partial and plenary indulgences can always be applied to the dead by way of suffrage, asking God to remit their sufferings if they are still in purgatory.