Incoercible

1. Not to be coerced; incapable of being compelled or forced.
2. (Physics) Not capable of being reduced to the form of a liquid by pressure; -- said of any gas above its critical temperature.
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.