2. (Physics) Not capable of being reduced to the form of a
liquid by pressure; -- said of any gas above its critical
point; -- also particularly of oxygen, hydrogen, nitrogen,
and carbon monoxide, formerly regarded as incapable of
liquefaction at any temperature or pressure.
3. (Physics) That can note be confined in, or excluded from,
vessels, like ordinary fluids, gases, etc.; -- said of the
imponderable fluids, heat, light, electricity, etc.