1. Concerning something happening at the same time: Virginia's grandfather clock and the old school clock were very synchronic in that both of them could be heard at the full hours at the same time!
2. In medicine, referring to the study of the natural history of a disease by its state and distribution in a population at one time: The inferences about the longitudinal course from such a synchronic study are warranted only under special conditions, notably that the longitudinal course of the disease is itself unchanging and that subjects in the sample are a representative sample of the survivors.