Diachronic refers to changes or developments over time. Its opposite or antonym is synchronous, which means happening at the same time or simultaneously. While diachronic is concerned with historical, evolutionary, or sequential changes, synchronous relates to immediate or contemporaneous events. Another antonym for diachronic is static, which means unchanging or constant. In contrast, diachronic is dynamic and progressive, exploring how things change and develop over time. Finally, another opposite of diachronic is timeless, which pertains to something that is eternal and unchanging, with no consideration for temporal alteration. In summary, antonyms for diachronic include synchronous, static, and timeless.