The servo motor is an engine, that can drive to different set point positions and then maintains these in the sense of a control drive. The position is thereby in case of a rotatory engine a angle position and in the case of a linear motor a distance position. Starting and maintaining the given position are made by a position control.
The term servo motor does not describe a physical action principle like for instance designations such as DC motor or Induction Machines, but the name makes from its earlier operational task as auxiliary drive (servus lat. slave). Generally however brushless direct current motors or vector-regular asynchronous/synchronous motors are meant with servo actuator.