gnomon

noun5

1. •
   An object such as a pillar or a rod that is used to tell time by the shadow it casts when the sun shines on it, especially the pointer on a sundial.

   • [T]he gnomon being the upright, and the shadow the side, the square-root of the sum of the squares is the hypotenuse; […] — 1817
2. •
   An object such as a pillar used by an observer to calculate the meridian altitude of the sun (that is, the altitude of the sun when it reaches the observer's meridian), for the purpose of determining the observer's latitude.
3. •
   The index of the hour circle of a globe.

   • Index of a globe, the little style or gnomon, which being fixed on the pole of the globe, and turning round with it, points out the hours upon the hour circle. — 1809
4. •
   A plane figure formed by removing a parallelogram from a corner of a larger parallelogram.
5. •
   A number representing the increment between two figurate numbers (“numbers equal to the numbers of dots in geometric figures formed of dots”).