WordNet records no synonym and no antonym for justification. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
something (such as a fact or circumstance) that shows an action to be reasonable or necessary
“he considered misrule a justification for revolution”
a statement in explanation of some action or belief
the act of defending or explaining or making excuses for by reasoning
“the justification of barbarous means by holy ends”
Synonyms, antonyms and the broader and narrower relations for justification come from Princeton University’s WordNet. Full sources and licences.