relation back 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.
(law) the principle that an act done at a later time is deemed by law to have occurred at an earlier time
“his attorney argued for the relation back of the amended complaint to the time the initial complaint was filed”
Synonyms, antonyms and the broader and narrower relations for relation back come from Princeton University’s WordNet. Full sources and licences.