Court martial, Union law and death sentences: the three heads of Article 72
Background
Article 72 gives the President the power to grant pardons, reprieves, respites or remissions of punishment, and to suspend, remit or commute a sentence. The power does not run over every conviction in the country. It reaches, first, all cases in which a Court Martial passed the punishment or sentence; second, all cases where the offence is against a law relating to a matter to which the Union's executive power extends; and third, all cases of a sentence of death. The Article then protects two other powers that sit alongside it. An officer of the armed forces of the Union on whom a law confers power to suspend, remit or commute a court martial sentence keeps that power. So does a Governor of a State who can suspend, remit or commute a death sentence under a law in force. Article 72 is therefore a clemency power confined to the three heads it lists, and it stands beside the Governor's own power rather than replacing it.
In plain words
Reprieve A stay of the execution of a sentence — It postpones the carrying out of a sentence without cancelling it.
Commutation The substitution of a lighter punishment — A sentence is replaced by one of a milder kind.
Court Martial A military court trying members of the armed forces — Article 72 covers its sentences, and a service officer's own power over them is kept intact.