accretion has 1 distinct synonym spread across 1 of its 6 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.
an increase by natural growth or addition
something contributing to growth or increase
“he scraped away the accretions of paint”
(astronomy) the formation of a celestial object by the effect of gravity pulling together surrounding objects and gases
(biology) growth by addition as by the adhesion of parts or particles
(geology) an increase in land resulting from alluvial deposits or waterborne sediment
(law) an increase in a beneficiary's share in an estate (as when a co-beneficiary dies or fails to meet some condition or rejects the inheritance)
Synonyms, antonyms and the broader and narrower relations for accretion come from Princeton University’s WordNet. Full sources and licences.