synchronous has 2 distinct synonyms spread across 1 of its 2 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 asynchronous.
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.
occurring or existing at the same time or having the same period or phase
“recovery was synchronous with therapy”
(digital communication) pertaining to a transmission technique that requires a common clock signal (a timing reference) between the communicating devices in order to coordinate their transmissions
Synonyms, antonyms and the broader and narrower relations for synchronous come from Princeton University’s WordNet. Full sources and licences.