What is Ignorance?

Legal Definition
Ignorance is the lack of knowledge. The word ignorant is an adjective describing a person in the state of being unaware and is often (incorrectly) used to describe individuals who deliberately ignore or disregard important information or facts.

Ignoramus is commonly used in the UK, Ireland, and the US as a term for someone who is willfully ignorant. Ignorance is distinguished from stupidity, although both can lead to "unwise" acts.

Writer Thomas Pynchon articulated about the scope and structure of one's ignorance: "Ignorance is not just a blank space on a person's mental map. It has contours and coherence, and for all I know rules of operation as well. So as a corollary to [the advice of] writing about what we know, maybe we should add getting familiar with our ignorance, and the possibilities therein for writing a good story."

The legal principle that ignorantia juris non excusat, literally "ignorance of the law is no excuse", stands for the proposition that the law applies also to those who are unaware of it.
-- Wikipedia
Legal Definition
The want of knowledge.

2. Ignorance is distinguishable from error. Ignorance is want of knowledge; error is the non-conformity or opposition of our ideas to the truth. Considered as a motive of our actions, ignorance differs but little from error. They are generally found together, and what is said of one is said of both.

3. Ignorance and error, are of several kinds. 1. When considered as to their object, they are of law and of fact. 2. When examined as to their origin, they are voluntary or involuntary, 3. When viewed with regard to their influence on the affairs of men, they are essential or non-essential.

4. - 1. Ignorance of law and fact. 1. Ignorance of law, consists in the want of knowledge of those laws which it is our duty to understand, and which every man is presumed to know. The law forbids any one to marry a woman whose husband is living. If any man, then, imagined he could marry such a woman, he would be ignorant of the law; and, if he married her, he would commit an error as to a matter of law. How far a party is bound to fulfil a promise to pay, upon a supposed liability, and in ignorance of the law, see 12 East, R. 38; 2 Jac. & Walk. 263; 5 Taunt. R. 143; 3 B. & Cresw. R. 280; 1 John. Ch. R. 512, 516; 6 John. Ch. R. 166; 9 Cowen's R. 674; 4 Mass. R. 342; 7 Mass. R. 452; 7 Mass. R. 488; 9 Pick. R. 112; 1 Binn. R. 27. And whether he can be relieved from a contract entered into in ignorance or mistake of the law. 1 Atk. 591; 1 Ves. & Bea. 23, 30; 1 Chan. Cas. 84; 2 Vern. 243; 1 John. Ch. R. 512; 2 John. Ch. R. 51; 1 Pet. S. C. R. 1; 6 John. Ch. R. 169, 170; 8 Wheat. R. 174; 2 Mason, R. 244, 342.

5. - 2. Ignorance of fact, is the want of knowledge as to the fact in question. It would be an error resulting from ignorance of a fact, if a man believed a certain woman to be unmarried and free, when in fact, she was a married woman; and were he to marry her under that belief, he would not be criminally responsible. Ignorance of the laws of a foreign government, or of another state; is ignorance of a fact. 9 Pick. 112. Vide, for the difference between ignorance of law and ignorance of fact, 9 Pick. R. 112; Clef. des Lois Rom. mot Fait; Dig. 22, 6, 7.

6. - 2. Ignorance is either voluntary or involuntary. 1. It is voluntary when a party might, by taking reasonable pains, have acquired the necessary knowledge. For example, every man might acquire a knowledge of the laws which have been promulgated, a neglect to become acquainted with them is therefore voluntary ignorance. Doct. & St. 1, 46; Plowd. 343.

7. - 2. Involuntary ignorance is that which does not proceed from choice, and which cannot be overcome by the use of any means of knowledge known to him and within his power; as, the ignorance of a law which has not yet been promulgated.

8. - 3. Ignorance is either essential or non-essential. 1. By essential ignorance is understood that which has for its object some essential circumstance so intimately connected with the: matter in question, and which so influences the parties that it induces them to act in the business. For example, if A should sell his horse to B, and at the time of the sale the horse was dead, unknown to the parties, the fact of the death would render the sale void. Poth. Vente, n. 3 and 4; 2 Kent, Com. 367.

9. - 2. Non-essential or accidental ignorance is that which has not of itself any necessary connexion with the business in question, and which is not the true consideration for enteting into the contract; as, if a man should marry a woman whom he believed to be rich, and she proved to be poor, this fact would not be essential, and the marriage would therefore be good. Vide, generally, Ed. Inj. 7; 1 Johns. h. R. 512; 2 Johns. Ch. R. 41; S. C. 14 Johns. R 501; Dougl. 467; 2 East, R. 469; 1 Campb. 134: 5 Taunt. 379; 3 M. & S. 378; 12 East, R. 38; 1 Vern. 243; 3 P. Wms. 127, n.; 1 Bro. C. C. 92; 10 Ves. 406; 2 Madd. R. 163; 1 V. & B. 80; 2 Atk. 112, 591; 3 P. Wms. 315; Mos. 364; Doct. & Stud. Dial. 1, c. 26, p. 92; Id. Dial. 2, ch. 46, p. 303; 2 East, R. 469; 12 East, R. 38; 1 Fonbl. Eq. B. 1, ch. 2, 7, note v; 8 Wheat. R. 174; S. C. 1 Pet. S. C. R. 1; 1 Chan. Cas. 84; 1 Story, Eq. Jur. 137, note 1; Dig. 22, 6; Code, 1, 16; Clef des Lois Rom. h. t.; Merl. R¬ępert. h. t.; 3 Sav. Dr. Rom. Appendice viii., pp. 337 to 444.
-- Bouviers Law Dictionary
Legal Definition
The want or absence of knowledge. Ignorance of law is want of knowledge or acquaintance with the laws of the land in so far as they apply to the act. relation, duty, or matter under consideration. Ignorance of fact is want of knowledge of some fact or facte constituting or relating to the subject-matter in hand. Marshall v. Coleman, 187 III. 556, 58 N. E. 628; Haven v. Foster, 9 Pick. (Mass.) 130, 19 Am. Dec. 353. Ignorance is not a slate Of the mind in the sense in which sanity and insanity are. When the mind is ignorant of a fact, its condition still remains sound ; the power of thinking, of judging, of willing, is just as complete before communication of the fact as after; the essence or texture, so to speak, of the mind, is not, as in the case of insanity, affected or impaired. Ignorance of a particular fact consists in this: that the mind, although sound and capable of healthy action, has never acted upon the fact in question, because the subject has never been brought to the notice of the perceptive faculties. Meeker v. Boylan, 28 N. J. Law, 274. Synonyms. "Ignorance" and "error" or "mistake" are not convertible terms. The former is a lack of information or absence of knowledge; the latter, a misapprehension or confusion of information, or a mistaken supposition of the possession of knowledge. Error as to a fact may imply ignorance of the truth; but ignorance does not necessarily imply error. Hutton v, Edgerton, 6 Rich. (S. Ct) 489; Culbreath v. Culbreath, 7 Ga. 70, 50 Am. Dec. 375. Essential ignorance is ignorance in relation to some essential circumstance so intimately connected with the matter in question, and which so influences the parties, that it induces them to act in the business. Both. Vente, nn. 3, 4; 2 Kent, Comm. 367. Non-essential or accidental ignorance is that which bas not of itself any necessary connection with the business in question, and which is not the true consideration for entering into the contract. Involuntary ignorance is that which does not proceed from choice, and which cannot be overcome by the use of any means of knowledge known to a person and within his power; as the ignorance of a law which has not yet been promulgated. Voluntary ignorance exists when a party might, by taking reasonable pains, have acquired the necessary knowledge. For example, every man might acquire a knowledge of the laws which have been promulgated. Doct. & Stud. 1, 46; Plowd. 348.
-- Black's Law Dictionary
Legal Definition
Want of knowledge. See 55 Am. St. Rep. 494, note.
-- Ballentine's Law Dictionary