Modification, exchange, or substitution.
Commutation is the replacement of a greater amount by something lesser. To commute periodic payments means to substitute a single payment for a number of payments, or to come to a "lump sum" settlement.
In Criminal Law, commutation is the substitution of a lesser punishment for a greater one. Contrasted with clemency, which is an act of grace eliminating a sentence or punishment, commutation is the modification or reduction of a punishment.
The change from consecutive prison sentences to concurrent sentences is a commutation of punishment.
n. the act of reducing a sentence resulting from a criminal conviction by the executive clemency of the Governor of the State, or President of the United States in the case of federal crimes. This is not the same as a pardon which wipes out the conviction or the actual or potential charge (as when President Gerald R. Ford pardoned ex-President Richard M. Nixon even without charges having been officially made--a rare instance). A pardon implies either that the conviction was wrong, that there has been complete rehabilitation of the party, or that he/she has lived an exemplary life for many years and deserves to have his/her name cleared in old age. Commutation implies the penalty was excessive or there has been rehabilitation, reform, or other circumstances such as good conduct or community service. Commutation is sometimes used when there is evidence that the defendant was not guilty but it would prove embarrassing to admit an outright error by the courts. (See: executive clemency, pardon)
COMMUTATION, punishments. The change of a punishment to which a person has been condemned into a less severe one. This can be granted only by the executive authority in which the pardoning power resides.