Real Dictionary

sundial
countable noun: A sundial is a device used for telling the time when the sun is shining. The shadow of an upright rod falls onto a flat surface that is marked with the hours, and points to the correct hour.

sundial in American English
an instrument that indicates time by the position of a gnomon's shadow cast by the sun on the face of a dial marked in hours
noun: an instrument that indicates the time of day by means of the position, on a graduated plate or surface, of the shadow of the gnomon as it is cast by the sun

sundial in British English
noun: a device indicating the time during the hours of sunlight by means of a stationary arm (the gnomon) that casts a shadow onto a plate or surface marked in hours