1. 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.
Source: from Federal Standard 1037C and from MIL-STD-188