An indult in Catholic canon law is a permission, or privilege, granted by the competent church authority - the Holy See or the diocesan bishop, as the case may be - for an exception from a particular norm of church law in an individual case.
For example, an indult is needed when members of the consecrated life want to be dispensed from their religious vows, or when priests and deacons voluntarily seek to return to the lay state (usually to marry). Another example: an apostolic indult is needed from the local ordinary for priestly or diaconal ordinations to be done within a year before the normal date; if the ordination is done more than one year in advance of the normal date, then a papal apostolic indult from the Holy See (through the Congregation for Catholic Education, the Congregation for the Clergy, or the Congregation for Religious) is also needed.
The best-known indult among lay Catholics in recent times was the one granted in 1984 by Pope John Paul II authorising the world's bishops to permit celebrations of the Tridentine Mass liturgy in their dioceses. This indult gave rise to the term "indult Catholics", referring to Catholics who attended such celebrations. This indult was superseded in 2007 by new legislation introduced by Pope Benedict XVI in the motu proprio Summorum Pontificum.