'Gnomon' definitions:

Definition of 'gnomon'

noun

Indicator provided by the stationary arm whose shadow indicates the time on the sundial

From: GCIDE

Gnomon \Gno"mon\, n. [L. gnomon, Gr. ? one that knows, the index of a sundial. See Gnome.] [1913 Webster]
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. [1913 Webster]
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. [1913 Webster]
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. [1913 Webster]
4. The index of the hour circle of a globe. Gnomonic