(1):

(a.) Not to be coerced; incapable of being compelled or forced.

(2):

(a.) 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):

(a.) That can note be confined in, or excluded from, vessels, like ordinary fluids, gases, etc.; - said of the imponderable fluids, heat, light, electricity, etc.