The synchro is a type of rotary electrical transformer that is used for measuring the angle of a rotating machine such as an antenna platform. The primary winding of the transformer, fixed to the rotor, is excited by a sinusoidal electric current, which by electromagnetic induction causes currents to flow in three star-connected secondary windings fixed at 120 degrees to each other on the stator. The relative magnitudes of secondary currents are measured and used to determine the angle of the rotor relative to the stator, or the currents can be used to directly drive an electric motor that will rotate in unison with the synchro. In the latter case, the whole device is also called a selsyn (a word made up from self-synchronizing).

The synchro was developed during World War II as part of a servomechanism that was used to synchronise the movement of a gun turret to a control dial.

Synchros designed for terrestrial use tend to be driven at 60 hertz (the mains frequency in many countries), while those for marine or aeronautical use tend to operate at 400 hertz (the frequency of the on-board generator driven by the engines).