From BiblePortal Wikipedia

Webster's Dictionary [1]

(1): ( n.) Hence, a race course; especially, the Olympic course for foot races.

(2): ( n.) 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.

(3): ( n.) A modern structure, with its inclosure, resembling the ancient stadium, used for athletic games, etc.

(4): ( n.) 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.

The Nuttall Encyclopedia [2]

The course on which were celebrated the great games (foot-racing, wrestling, &c.) of ancient Greece, held at Olympia, Athens, and other places; the most famous was that laid out at Olympia; length 600 Greek feet, which was adopted as the Greek standard of measure, and equalled 606½ English feet.