incoercible

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Difficult or impossible to coerce or control forcibly.

from The Century Dictionary.

• Not to be coerced or compelled; incapable of being constrained or forced.
• In physics, incapable of being reduced to a liquid form by any amount of pressure. Certain gases were formerly supposed to have this property. See gas.
• In physical:
• Incapable of reduction to tangible condition by pressure: applied to forms of energy, such as heat and electricity, when they were thought of as extremely subtile fluids.

from the GNU version of the Collaborative International Dictionary of English.

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

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective Not to be coerced; incapable of being compelled or forced.
• adjective physics, of a gas Not capable of being reduced to liquid form by pressure.
• adjective physics, archaic That cannot be confined in, or excluded from, vessels, like ordinary fluids, gases, etc.; said of heat, light, electricity, etc.