The part of a hard disk to which the read/write arms attaches. All the heads are attached to a single head actuator, also called an actuator arm, that moves the heads around the platters. Older hard drives used a stepper motor actuator, which moved the heads based on a motor reacting to stepper pulses. Modern hard drives use a voice coil actuator, which controls the movement of a coil toward or away from a permanent magnet based on the amount of current flowing through it. This guidance system is called a servo.
See also Webopedia's "Did You Know... What's Inside a Hard Drive?"