asynchronous has no recorded synonym. 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 synchronous.
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.
(digital communication) pertaining to a transmission technique that does not require a common clock between the communicating devices; timing signals are derived from special characters in the data stream itself
not synchronous; not occurring or existing at the same time or having the same period or phase
Synonyms, antonyms and the broader and narrower relations for asynchronous come from Princeton University’s WordNet. Full sources and licences.