1. An act that is not authorized by law.
2. The state of not being legally authorized.
“A contract made ultra vires is void; but not [strictly speaking] on the ground of illegality. Lord Cairns … takes exception to the use of the term ‘illegality,’ pointing out that it is not the object of the contracting parties, but the incapacity of one of them, that avoids the contract.” William R. Anson, Principles of the Law of Contract 190 (Arthur L. Corbin ed., 3d Am. ed. 1919).
“It must not be thought that illegality in the law of contract is co-terminous with illegality in the criminal law, for a contract may be illegal without involving any breach of the criminal law at all.” P.S. Atiyah, An Introduction to the Law of Contract 257 (3d ed. 1981).
3. The state or condition of being unlawful. • The affirmative defense of illegality must be expressly set forth in the response to the opponent’s pleading. Fed. R. Civ. P. 8(c).
