The mechanism on a hard drive, floppy drive, or optical drive that attaches to the read/write arms. The actuator is controlled by an electrical signal that instructs it to extend or retract the arms so that they can move across the tracks on the drive platters. The movement of these lightweight arms is very fast, appearing as a tiny flicker to the human eye.
In engineering, actuators are a subdivision of transducers. They are devices that transform an input signal (usually electrical) into motion.