When the word stadium was first used in the English language in the late fourteenth century, it actually referred to a unit of measurement used in the ancient world, equivalent to one-eighth of a Roman mile. That was the standard length used in competitive footraces, and it came to be applied to running tracks because they were made to be one stadium in length. Eventually, that got naturally extended to any open-air circular structure for viewing sporting events. Going back in time, stadium was a Latin word that came from Greek stadion, which was pretty confusing because it could be used to refer to several different types of length, but especially the running track in Olympia. Beyond that, it might be from the Proto-Indo-European root steh, meaning "stand", on the notion of it being a "fixed" length.
Hello! I'm Adam Aleksic, a sophomore studying government and linguistics at Harvard University, where I co-founded the Harvard Undergraduate Linguistics Society. In addition to etymology, I also really enjoy politics, vexillology, geography, board games, conlanging, and law.