Gnomon, gnomonis, m. gen. Plin. The tooth whereby the age of an horse is knowne. That which sheweth the houres in a diall. A rule to knowe any thing by.
Lewis and Short: Latin dictionary
gnōmon, ŏnis, m., = gnw/mwn, the pin or style of a sundial, the gnomon, Plin. 2, 72, 74, 182; Vitr. 1, 6; 9, 8; Mart. Cap. 6, 194.