n.1 the rod or pin etc. on a sundial that shows the time by the position of its shadow.
2 Geom. the part of a parallelogram left when a similar parallelogram has been taken from its corner.
3 Astron. a column etc. used in observing the sun's meridian altitude.
Derivatives:
gnomonic adj.
Etymology: F or L gnomon f. Gk gnomon indicator etc. f. gignosko know