the raised part of a sundial that casts the shadow; a style.
an early astronomical instrument consisting of a vertical shaft, column, or the like, for determining the altitude of the sun or the latitude of a position by measuring the length of its shadow cast at noon.
Geometry. (formerly) the part of a parallelogram that remains after a similar parallelogram has been taken away from one of its corners.
"vertical shaft that tells time by the shadow it casts" (especially the triangular plate on a sundial), 1540s, from Latin gnomon, from Greek gnomon "indicator," literally "one who discerns," from gignoskein "to come to know" (see gnostic (adj.)).