decoration has 7 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.
something used to beautify
an award for winning a championship or commemorating some other event
the act of decorating something (in the hope of making it more attractive)
Synonyms, antonyms and the broader and narrower relations for decoration come from Princeton University’s WordNet. Full sources and licences.