A servo-mechanism, sometimes shortened to servo, was a mechanical device in Data's neck, designed to approximate Human movements.
When Beverly Crusher, under the influence of a mind-altering game, disabled Data, she lied that he had entered Sickbay complaining of a servo malfunction. (TNG: "The Game")
When Timothy was pretending to be an android in order to suppress negative emotions surrounding the death of his parents, he tried to emulate the unique movements of Data's head produced by the latter's servo-mechanisms. However, he did it excessively and as a result did not appear much like Data and made it difficult for Data to style Timothy's hair. When Data told Timothy that the latter's head movements were impeding his ability to style his hair, Timothy explained that he was attempting to emulate Data. Data explained about the servo-mechanisms and their purpose and noted to himself that he did not realize that their effect was so "distracting". Later, Timothy claimed he had servo-mechanisms in his mouth to justify having yawned. (TNG: "Hero Worship")
Sometime before 2370, Data attempted to swim in Devala Lake while sailing with Geordi La Forge but ended up sinking to the bottom of the lake and needing to walk to the shore. As a result, his servos became saturated with water and it took nearly two weeks to dry them out. (TNG: "Descent, Part II")