In general:
Drive elements activated by an amplifying auxiliary force (e.g. power steering, servo brake).
In fluid technology:
Collective term for an automatic drive system, which contains a power amplifier and a feedback to change the output variable as a defined function of an input variable.
In terms of devices, a servo drive consists of a combination of servo valvehydraulic motor (servo motor), servo valve hydraulic cylinders (servo cylinder), but the combination of a servo pump and normal cylinder/motor is also designated as the servo drive.