1) one who stands in the front rank
1a) a leader, chief, champion
Part of Speech: noun masculine
From G4413 and G2476; one standing first in the ranks, that is, a captain (champion): - ringleader.
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 πρωτοστάτης; click on the second column to search for that grammatical form of the root πρωτοστάτης; click on the third column to search for that word and grammatical form; click on the total to search for the root πρωτοστάτης.