Where does “στάδιον” come from?

στάδιον (Ancient Greek) comes from Ancient Greek στάδιος, from Proto-Indo-European *steh₂-.

στάδιον (Ancient Greek): A 600-foot track for footraces and the...

Definitions

Step	Language	Word	Meaning
1	Ancient Greek	στάδιος	standing fast and firm; firm, fixed, immobile;...
2	Proto-Indo-European	*steh₂-	—

στάδιον — Etymology, Origin & Word History | Etymology Explorer