A heliostat is a device that includes a mirror, which turns so as to keep reflecting sunlight toward a predetermined target, compensating for the sun's apparent motions in the sky. The support structure of a heliostat typically consists of a foundation, a stand and a reflector frame holding the mirror and giving it the required shape.
Large installations such as solar thermal power plants include fields of heliostats. Usually, all the mirrors in such a field are controlled by a complex computer system that the resulting reflection is pointing onto the receiver target. In a solar tower plant this target is the aperture or absorber of a receiver installed on top of the tower above the heliostat field.
Heliostats are one of the most modular elements of any solar tower plant and the heliostat field is the most expensive component. Heliostats should be distinguished from solar trackers or sun-trackers that point directly at the sun.