A Greek measure of length, being the chief one used for
itinerary distances, also adopted by the Romans for nautical and
astronomical measurements. It was equal to 600 Greek or 625 Roman feet,
or 125 Roman paces, or to 606 feet 9 inches English. This was also
called the Olympic stadium, as being the exact length of the foot-race
course at Olympia.

Hence, a race course; especially, the Olympic course for
foot races.

A kind of telemeter for measuring the distance of an
object of known dimensions, by observing the angle it subtends;
especially (Surveying), a graduated rod used to measure the distance of
the place where it stands from an instrument having a telescope, by
observing the number of the graduations of the rod that are seen
between certain parallel wires (stadia wires) in the field of view of
the telescope; – also called stadia, and stadia rod.