grace period has 1 distinct synonym spread across 1 of its 1 sense. 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.
a period of time past the deadline for fulfilling an obligation during which a penalty that would be imposed for being late is waived, especially an extended period granted as a special favor
“The payment had originally been due on April 1 but we had a grace period which expired in June.”
Synonyms, antonyms and the broader and narrower relations for grace period come from Princeton University’s WordNet. Full sources and licences.