What is a Gnomon?

[ˈnoʊmɒn] NO-mon, from Greek γνώμων, gnōmōn, literally "one that knows or examines"

1. The raised part of a sundial that casts the shadow; a style.

2. 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.

3. Geometry. (formerly) the part of a parallelogram that remains after a similar parallelogram has been taken away from one of its corners.