Real Dictionary

natural gas
a mixture of gaseous hydrocarbons, chiefly methane, occurring naturally in the earth, often in association with petroleum deposits, and piped to cities, factories, etc., to be used as a fuel
noun: a combustible mixture of gaseous hydrocarbons that accumulates in porous sedimentary rocks, esp. those yielding petroleum, consisting usually of over 80 percent methane together with minor amounts of ethane, propane, butane, nitrogen, and, sometimes, heli
noun: a gaseous mixture consisting mainly of methane trapped below ground; used extensively as a fuel
uncountable noun: Natural gas is gas which is found underground or under the sea. It is collected and stored, and piped into people's homes to be used for cooking and heating.

natural gas in Mechanical Engineering
noun: Natural gas is a fuel consisting mostly of methane gas, extracted from under the ocean or ground.