\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.