According to the oxford English Dictionary gas is;
An air-like fluid substance which expands freely to fill any space available, irrespective of its quantity:
The word originated from the Greek word khoas meaning chaos
Sometime in the 1600's Belgian chemist J. B. van Helmont (1577–1644), used it to denote an occult principle which he believed to exist in all matter with Dutch g representing Greek kh .
Modern plumbers, heating engineers and "gas" engineers use the term to refer exclusively to the fossil fuel methane gas. A hydrocarbon chemical formula CH 4. Likewise this website always refers to methane as gas. Also know as natural gas.
Properties of Gas;
Methane is a chemical compound with the chemical formula CH 4. It is the simplest alkane and the main component of natural gas. Wikipedia
Technically gas is not explosive. It is the air gas mix which is explosive as combustion is not possible without oxygen.
Density: 0.66 kg/m³
Boiling point: -164 °C
Molar mass: 16.04 g/mol
Melting point: -182 °C
Soluble in: Water
IUPAC ID: Tetrahydridocarbon, Methane
Other hydro carbons referred to as gas;
Gasoline or petrol (US English) oddly a liquid rather than a gas.
Town gas, a gas manufactured from coal once used to supply urban homes hence the name. To differentiate form this methane is often called natural gas. One of the reasons for the switch to methane was safety as methane is less explosive.