excuse has 15 distinct synonyms spread across 8 of its 9 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
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.
defend, explain, clear away, or make excuses for by reasoning
“rationalize the child's seemingly crazy behavior”
a defense of some offensive behavior or some failure to keep a promise etc.
“he kept finding excuses to stay”
a note explaining an absence
“he had to get his mother to write an excuse for him”
Synonyms, antonyms and the broader and narrower relations for excuse come from Princeton University’s WordNet. Full sources and licences.