What is Methane?

Legal Definition
Methane (US /ˈmɛθn/ or UK /ˈmθn/) is a chemical compound with the chemical formula CH4 (one atom of carbon and four atoms of hydrogen). It is a group 14 hydride and the simplest alkane, and is the main constituent of natural gas. The relative abundance of methane on Earth makes it an attractive fuel, though capturing and storing it poses challenges due to its gaseous state under normal conditions for temperature and pressure.

Natural methane is found both below ground and under the sea floor. When it reaches the surface and the atmosphere, it is known as atmospheric methane. The Earth's atmospheric methane concentration has increased by about 150% since 1750, and it accounts for 20% of the total radiative forcing from all of the long-lived and globally mixed greenhouse gases (these gases don't include water vapor which is by far the largest component of the greenhouse effect).
-- Wikipedia
Legal Definition
Flammable hydrocarbon gas that is colorless and odorless. Natural gas's main component, about 75 to 99 percent. Non-toxic, it is an asphyxiant and a major hothouse gas. Catalytic oxidation converts methane into methanol.