Webster's English Dictionary

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petroleum

pe.tro.le.um \p*-'tro--le--*m, -'tro-l-y*m\ n [ML, fr. L petr- + oleum oil 
   - more at OIL] 1: an oily flammable bituminous liquid that may vary from 
   almost colorless  to black, occurs in many places in the upper strata of 
   the earth, is a complex mixture of hydrocarbons with small amounts of other 
   substances, and is prepared for use as gasoline, naphtha, or other products 
   by various refining processes 2: a substance similar in composition to 
   petroleum