GNOMON

noun Etymology: Latin, from Greek gnōmōn interpreter, pointer on a sundial, from gignōskein Date: 1546 an object that by the position or length of its shadow serves as an indicator especially of the hour of the day: as, the pin of a sundial, a column or shaft erected perpendicular to the horizon, the remainder of a parallelogram after the removal of a similar parallelogram containing one of its corners