Definition from Nolo’s Plain-English Law Dictionary
Reducing a sentence resulting from a criminal conviction, which can be done by the governor of a state (state convictions) or the president of the United States (federal convictions). A commutation is distinguished from a pardon, which wipes out the conviction and can even stop 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 of the use of the presidential pardon power). Commutation implies the penalty was excessive or there is evidence of rehabilitation, reform, community service, or other indications of good conduct.