Pentecost = "the fiftieth day"
1) the second of the three great Jewish feasts, celebrated at Jerusalem yearly, the seventh week after the Passover, in grateful recognition of the completed harvest
Part of Speech: noun feminine
Citing in TDNT: 6:44, 826
Feminine of G4004; fiftieth (G2250 being implied) from Passover, that is, the festival of "pentecost": - Pentecost.
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 πεντηκοστή; click on the second column to search for that grammatical form of the root πεντηκοστή; click on the third column to search for that word and grammatical form; click on the total to search for the root πεντηκοστή.