Focus-by-wire
A manual focus ring that sends an electronic signal to a motor rather than moving the optics mechanically.
In depth
Because there is no mechanical linkage there are usually no hard stops and no repeatable distance scale, and on some lenses the response is speed-sensitive — turning quickly moves further than turning slowly through the same angle. That makes repeatable focus pulls difficult, which is why cine lenses retain a mechanical coupling.
Examples in practice
A focus-by-wire lens cannot be preset to a marked distance before a shot, since the ring has no fixed relationship to the optics.