Merriam-Webster Collegiate Dictionary - gnomon

 noun  Etymology: Latin, from Greek gnomon interpreter, pointer on a sundial, from gignoskein  Date: 1546  1. an object that by the position or length of its shadow serves as an indicator especially of the hour of the day: as  a. the pin of a sundial  b. a column or shaft erected perpendicular to the horizon  2. the remainder of a parallelogram after the removal of a similar parallelogram containing one of its corners