The Collaborative International Dictionary of English v.0.48:

Petroleum \Pe*tro"le*um\ (p[-e]*tr[=o]"l[-e]*[u^]m), n. [NL.,
   fr. L. petra a rock + oleum oil: cf. F. p['e]trole. Cf.
   Petrify, and Oil.]
   Rock oil, mineral oil, or natural oil, a dark brown or
   greenish inflammable liquid, which, at certain points, exists
   in the upper strata of the earth, from whence it is pumped,
   or forced by pressure of the gas attending it. It consists of
   a complex mixture of various hydrocarbons, largely of the
   methane series, but may vary much in appearance, composition,
   and properties. It is refined by distillation, and the
   products include kerosene, benzine, gasoline, paraffin, etc.
   [1913 Webster]

   Petroleum spirit, a volatile liquid obtained in the
      distillation of crude petroleum at a temperature of
      170[deg] Fahr., or below. The term is rather loosely
      applied to a considerable range of products, including
      benzine and ligroin. The terms petroleum ether, and
      naphtha, are sometimes applied to the still more volatile
      products, including rhigolene, gasoline, cymogene, etc.
      [1913 Webster] Petroleur

WordNet (r) 3.0 (2006):

petroleum
    n 1: a dark oil consisting mainly of hydrocarbons [syn:
         petroleum, crude oil, crude, rock oil, fossil
         oil, oil]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "petroleum":
   avgas, benzine, coal oil, crude, crude oil, electricity, ethyl,
   ethyl gas, fossil oil, gas, gasoline, high-octane gas, high-test,
   illuminant, illuminating gas, kerosene, lead-free gas,
   light source, low-lead gas, luminant, motor oil, oil, paraffin,
   petrol, premium gas, regular, rock oil



U.S. Gazetteer Counties (2000):

Petroleum -- U.S. County in Montana
   Population (2000):    493
   Housing Units (2000): 292
   Land area (2000):     1653.903684 sq. miles (4283.590695 sq. km)
   Water area (2000):    20.110830 sq. miles (52.086808 sq. km)
   Total area (2000):    1674.014514 sq. miles (4335.677503 sq. km)
   Located within:       Montana (MT), FIPS 30
   Location:             47.000478 N, 108.290051 W
   Headwords:
    Petroleum
    Petroleum, MT
    Petroleum County
    Petroleum County, MT