In the words of Euclid:
- And in any parallelogrammic area let any one whatever of the parallelograms about its diameter with the two complements be called a gnomon.
The word gnomon is a Greek one, generally meaning marker or indicator (literally: a thing that enables something to be known), and specifically meaning the piece of a sundial that casts the shadow.
Some sources interpret gnomon to mean carpenter's square.
The words gnostic and agnostic come from the same root.