Source : Webster's Revised Unabridged Dictionary (1913)

Gnomon \Gno"mon\, n. [L. gnomon, Gr. ? one that knows, the index
   of a sundial. See {Gnome}.]
   1. (Dialing) The style or pin, which by its shadow, shows the
      hour of the day. It is usually set parallel to the earth's
      axis.

   2. (Astron.) A style or column erected perpendicularly to the
      horizon, formerly used in astronomocal observations. Its
      principal use was to find the altitude of the sun by
      measuring the length of its shadow.

   3. (Geom.) The space included between the boundary lines of
      two similar parallelograms, the one within the other, with
      an angle in common; as, the gnomon bcdefg of the
      parallelograms ac and af. The parallelogram bf is the
      complement of the parallelogram df.

   4. The index of the hour circle of a globe.

Source : WordNet®

gnomon
     n : indicator provided by the stationary arm whose shadow
         indicates the time on the sundial