A sequence of events is isochronous if the events occur regularly, or at equal time intervals. The term isochronous is used in several technical contexts, but usually refers to the primary subject maintaining a constant period or interval (the reciprocal of frequency), despite variations in other measurable factors in the same system.
Isochronous timing differs from synchronous timing, in that the latter refers to relative timing between two or more sequences of events.
In horology, a mechanical clock or watch is isochronous if it runs at the same rate regardless of changes in its drive force, so that it keeps correct time as its mainspring unwinds or chain length varies. This includes pendulum clocks. For example, as noted by Galileo Galilei in the late 16th century, the oscillation period of a given pendulum is nearly constant, regardless of the angle of its swing (if the angle is kept small). Isochrony is important in timekeeping devices.
In telecommunications, an isochronous signal is one where the time interval separating any two corresponding transitions is equal to the unit interval or to a multiple of the unit interval; but phase is arbitrary and potentially varying.
The term is also used in data transmission to describe cases in which corresponding significant instants of two or more sequential signals have a constant phase relationship.