From Envirowiki

Gas is one of the three major states of matter (the others being liquid, and solid). Gaseous matter is a highly excited fluid, and generally lighter than liquid. Gasses are easily compressible.

Gas often refers to Liquid Petroleum Gas, or Liquid Natural Gas, and in the United States it also refers to petroleum.

The Earth's atmosphere is made up largely of gasses, although it also contains water vapour, and birds and insects. The most common gasses in the atmosphere are nitrogen, oxygen, carbon dioxide, and trace elements.