Gnomon

Gnomon is the Greek word for the style of a sundial, or any object, commonly a vertical column, the shadow of which was observed in former times in order to learn the altitude of the sun, especially when on the meridian.

The art of constructing a gnomon sundial is sometimes termed gnomonics.

In geometry, a gnomon is a plane figure formed by removing a parallelogram from a corner of a larger parallelogram

Gnomonic projection is a projection of a sphere in which the centre of sight is the centre of the sphere.

To Pythagoras and the Pythagoreans a gnomon was a basic unit from which a number pattern can be generated. This is described in Figurate numbers, in which squares of numbers are generated from the gnomon of an Odd number.

Reference

Gnomons, from Pharoahs to Fractals, Midhat J. Gazalé, Princeton University Press, Princeton, 1999.