n. In astronomy, an instrument for showing when the sun arrives at the solstitial points.n. A mirror arranged with a telescope and sights so as to flash a reflection of the sun to a great distance. The instrument is used in geodetic triangulation to mark a station. See heliograph, 1.n. A plant of the genus Heliotropium, of the natural order Boraginaceæ.n. The bluish-purple or pinkish-lilac color of some flowers of the heliotrope.n. A mineral, a subspecies of quartz, of a deep-green color, peculiarly pleasant to the eye.n. A direct coal-tar color of the disazo type, derived from dianisidine. It dyes unmordanted cotton reddish violet in an alkaline salt bath.