heliostat
A device that includes a plane mirror which turns so as to keep reflecting sunlight toward a predetermined target, compensating for the sun's apparent motions in the sky. The target may be a physical object, distant from the heliostat, or a direction in space, and is almost always stationary relative to the heliostat, so the light is reflected in a fixed direction.
Each heliostat can be directed to bathe a particular park bench or tree in a beam of light. […] The three heliostats cost $355,000 together, not apiece.
Where it comes from
First use appear c. 1747, from New Latin heliostata, from Ancient Greek ἥλιος (hḗlios, “sun”) + Latin status (“stationary”). By surface analysis, helio- + -stat.
Adapted from the Wiktionary entry heliostat (CC BY-SA 4.0).