late 14c., "a foot race; an ancient measure of length," from Latin stadium "a measure of length; a course for foot-racers" (commonly one-eighth of a Roman mile or a little over 600 English feet; translated in early English Bibles by furlong), from Greek stadion "a measure of length; a race-course, a running track," especially the track at Olympia, which was one stadion in length. The meaning "running track," recorded in English from c. 1600, was extended to mean in modern-day context "large, open oval structure with tiers of seats for viewing sporting events" (1834).
"Originally the distance between successive stations of the shouters and runners employed to estimate distances" [Century Dictionary]. According to Barnhart, the Greek word might literally mean "fixed standard of length" (from stadios "firm, fixed," from PIE root *sta- "to stand"), or it may be from spadion, from span "to draw up, pull," with form influenced by stadios.