from The American Heritage® Dictionary of the English Language, 4th Edition
- n. An instrument in which a mirror is automatically moved so that it reflects sunlight in a constant direction. It is used with a pyrheliometer to make continuous measurements of solar radiation.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. 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.
from the GNU version of the Collaborative International Dictionary of English
- n. An instrument consisting of a mirror moved by clockwork, by which a sunbeam is made apparently stationary, by being steadily directed to one spot during the whole of its diurnal period; also, a geodetic heliotrope.
from The Century Dictionary and Cyclopedia
- n. An instrument consisting of a mirror carried by clockwork in such a way as to reflect the sun's rays in a fixed direction. The name is also improperly applied to a porte-lumière.
Circa 1750, from New Latin heliostata, from Ancient Greek ἥλιος (helios, "sun") + Latin status ("stationary"). (Wiktionary)