The state of being synchronous. 2. For repetitive events with the same, multiple, or submultiple repetition rates, a relationship among the events such that a significant instant of one event bears a fixed time relationship to a corresponding instant in another event. Note: Synchronism is maintained when there is a fixed, i.e., constant, phase relationship among the group of repetitive events. 3. The simultaneous occurrence of two or more events at the same instant on the same coordinated time scale.