adoption has 4 distinct synonyms spread across 2 of its 3 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.
the act of accepting with approval; favorable reception
“its adoption by society”
a legal proceeding that creates a parent-child relation between persons not related by blood; the adopted child is entitled to all privileges belonging to a natural child of the adoptive parents (including the right to inherit)
the appropriation (of ideas or words etc) from another source
“the borrowing of ancient motifs was very apparent”
Synonyms, antonyms and the broader and narrower relations for adoption come from Princeton University’s WordNet. Full sources and licences.