stadium: [16] Greek stádion denoted a ‘racetrack’, particularly the one at Olympia, which was about 185 metres long. In due course the word came to be used as a term for a measure of length equal to this, and that was the sense in which English originally acquired it, via Latin stadium. The original ‘racetrack’ was introduced in the 17th century, and ‘sports arena’ is a modern development of this. The Greek word itself was an alteration of an earlier spádion ‘racetrack’, a derivative of span ‘pull’ (source of English spasm). The change from sp- to st- was perhaps set in motion by Greek stádios ‘fixed, firm’. => spasm
