The word 'servo' means more than just those little RC Servo Motors we usually think of. Servo is a general term for a closed loop control system using negative feedback.
The cruise control in a car is one example of a servo system. It measures your speed and feeds that back into a control circuit which adjusts the accelerator to maintain speed.
For the familiar RC Servo motor, the position of the output shaft is measured and fed back to the internal control circuit which adjusts current to the motor to maintain position.