adjective

1. happening at the same time; occurring together; simultaneous
2. having the same period between movements, occurrences, etc.; having the same rate and phase, as vibrations
3. Elec. designating or of a machine, specif. an alternating-current motor, generator, or converter, whose normal operating speed is exactly proportional to the frequency of the current in the circuit to which it is connected

Etymology: LL synchronus < Gr synchronos: see synchronize