"Decretum" is a Latin word that means "a decree" or "an official order." Antonyms for this word include "revoke," "cancel," "nullify," "retract," "abolish," and "rescind." These words refer to actions that undo or invalidate an official order or decree. For example, a court might revoke a decree if they discover new evidence that changes their decision. Similarly, a government might rescind a decree if it is found to be ineffective or unpopular. In short, antonyms for "decretum" are words that describe the opposite action to an official decree, which is to revoke or cancel it.