The Collaborative International Dictionary of English v.0.48:

Onondagas \On`on*da"gas\, n. pl.; sing. Onondaga. (Ethnol.)
   A tribe of Indians formerly inhabiting what is now a part of
   the State of New York. They were the central or head tribe of
   the Five Nations.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Onondaga
    n 1: a member of the Iroquoian people formerly living between
         Lake Champlain and the Saint Lawrence River
    2: the Iroquoian language spoken by the Onondaga

U.S. Gazetteer Counties (2000):

Onondaga -- U.S. County in New York
   Population (2000):    458336
   Housing Units (2000): 196633
   Land area (2000):     780.293445 sq. miles (2020.950659 sq. km)
   Water area (2000):    25.400019 sq. miles (65.785744 sq. km)
   Total area (2000):    805.693464 sq. miles (2086.736403 sq. km)
   Located within:       New York (NY), FIPS 36
   Location:             43.048433 N, 76.179117 W
   Headwords:
    Onondaga
    Onondaga, NY
    Onondaga County
    Onondaga County, NY