1) Difficulty 2) Impracticableness 3) Infeasibility 4) Unfeasibility 
The doctrine of impracticability in the common law of contracts excuses performance of a duty, where that duty has become unfeasibly difficult or expensive for the party who was to perform. Impracticability is similar in some respects to the doctrine of impossibility because it is triggered by the occurrence of a condition which prevents one party...
• (n.) Intractableness; stubbornness. • (n.) An impracticable thing. • (n.) The state or quality of being impracticable; infeasibility.
Im·prac`ti·ca·bil'i·ty noun ; plural Impracticabilities 1. The state or quality of being impracticable; infeasibility. Goldsmith. 2. An impracticable thing. 3. Intractableness; stubbornness. 
[n] - the quality of not being usable
impracticableness noun the quality of not being usable