impossible

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

• adjective Incapable of having existence or of occurring.
• adjective Not capable of being accomplished.
• adjective Unacceptable; intolerable.
• adjective Extremely difficult to deal with or tolerate.

from The Century Dictionary.

• Not possible; non-existent or false by necessity.
• In law, in a stricter sense, prevented only by the act of God or a public enemy.
• Excessively odd; not to have been imagined; such as would not have been thought possible: as, she is a most impossible person; he wears an impossible hat.
• noun An impossibility.

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

• adjective Not possible; incapable of being done, of existing, etc.; unattainable in the nature of things, or by means at command; insuperably difficult under the circumstances; absurd or impracticable; not feasible.
• adjective (Math.) an imaginary quantity. See Imaginary.
• noun obsolete An impossibility.

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

• adjective Not possible, not able to be done.
• adjective colloquial, of a person Very difficult to deal with.
• noun an impossibility
• noun uncountable (with definite article) that which seems impossible

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun something that cannot be done
• adjective not capable of occurring or being accomplished or dealt with
• adjective used of persons or their behavior
• adjective totally unlikely