1) a space or distance of about 600 feet (185 m)
2) a race course
2a) place in which contests in running were held, the one who outstripped the rest and reached the goal first, receiving the prize. Courses of this description were found in most of the larger Greek cities, and were like that at Olympia, 600 Greek feet in length
Part of Speech: noun neuter
Or the masculine plural form, στάδιος stadios stad'-ee-os.
From the base of G2476 (as fixed); a stade or certain measure of distance; by implication a stadium or race course: - furlong, race.
These two sites give similar information, with the definition from several dictionaries and statistics on the use of the word.
Perseus Digital Library
University of Chicago's Logion lexicon
Thayer's dictionary plus other information.
From this site's dictionary (in Italian)
Click on the first column to search for that word as a form of the root στάδιον; click on the second column to search for that grammatical form of the root στάδιον; click on the third column to search for that word and grammatical form; click on the total to search for the root στάδιον.