assembly has 5 distinct synonyms spread across 3 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. Its one recorded opposite is disassembly.
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 group of machine parts that fit together to form a self-contained unit
the act of constructing something (as a piece of machinery)
a group of persons who are gathered together for a common purpose
a unit consisting of components that have been fitted together
the social act of assembling
“they demanded the right of assembly”
Synonyms, antonyms and the broader and narrower relations for assembly come from Princeton University’s WordNet. Full sources and licences.