1. Referring to things that exist or happen at the same time; coincident in time; simultaneous: Jane's return to health was synchronous with the medicine she was taking.
2. Relating to different events or things belonging to the same time or period; involving or indicating a contemporaneous or simultaneous occurrence: Mary's visit with her friend was synchronous with her brother being at their aunt's house. That meant that both of them were not at home during the morning hours.
3. Regarding something recurring at the same successive instants of time; keeping time with; going on at the same rate and exactly together; having coincident periods, as two sets of vibrations: The couple watched the synchronous movements of the bird's wings when it was flying over the lake.
4. Of a satellite: concerning the rotation round the parent planet at the same rate as the planet rotates: The orbit of the spacecraft seemed to be synchronous with that of the Earth.