A modern stadium (plural stadiums/stadia) is a place or venue for (mostly) outdoor sports, concerts, or other events and consists of a field or stage either partly or completely surrounded by a structure designed to allow spectators to stand or sit and view the event.)

Pausanias noted that for about half a century the only event at the ancient Greek Olympic festival was the race that comprised one length of the stade at Olympia, where the word "stadium" originated.

"Stadium" comes from the Greek word "stadion" (στάδιον), a measure of length equalling the length of 600 human feet. As feet are of variable length the exact length of a stadion depends on the exact length adopted for 1 foot at a given place and time. Although in modern terms 1 stadion = 600 ft (180 m), in a given historical context it may actually signify a length up to 15% larger or smaller.

A Roman stadium was defined somewhat differently to be a distance of 125 passus (double-paces), equal to about 185 m (607 ft).

Although most dictionaries provide for both "stadiums" and "stadia" as valid plurals, etymological sticklers sometimes apply "stadia" only to measures of length in excess of 1 stadium. (That the "stadium" measurement is used only in historical contexts perhaps explains the sustained use of the archaic plural.)