What is Undue Influence?

Legal Definition
In jurisprudence, undue influence is an equitable doctrine that involves one person taking advantage of a position of power over another person. This inequity in power between the parties can vitiate one party's consent as they are unable to freely exercise their independent will.
-- Wikipedia
Legal Definition
In contract law, a defense that can be used by a party to argue against the formation of a binding contract between two parties. The use of undue influence by one party over another puts the free will of one of the parties entering the contract into question, and therefore leads to the contract being unenforceable and voidable by the victim party. To prove undue influence, a party must show that one party to the contract is a person with weaknesses which make him likely to be affected by such persuasion, and that the party exercising the persuasion is someone in a special relationship with the victim that makes the victim especially susceptible to such persuasion.
Legal Definition
In regaild to the making of a wlll and other such matters, undue influence is persuasion carried to the point of overpowering the will or such a control over the person in question as prevents him from acting intelligently, understandingly, and voluntarily, and in effect destroys his free agency, and constrains him to do what he would not have done if such control had not been exercised. See Mitchell v. Mitchell, 43 Minn. 73, 44 N. W. 885; Bennett v. Bennett, 50 N. J. Eq. 439, 26 All. 573; Francis v. Wilkinson, 147 111. 370, 35 N. E. 150; Conley v. Nailer, 118 U. S. 127, 6 Sup. Ct. 1001, 30 In Ed. 112; Marx v. Mc-Glynn, 88 N. Y. 370; In re Logan's Estate, 195 Pa. 282, 45 Atl. 729; Mooney v. Olsen, 22 Kan. 79; In re Black's Estate, Myr. Prob. (Cal.) 31. Undue influence consists

(1) in the use, by one in whom a confidence is reposed by another, or who holds a real or apparent authority over him, of such confidence or authority, for the purpose of obtaining an unfair advantage over him;

(2) in taking an unfair advantage of another's weakness of mind; or

(3) in taking a grossly oppressive and unfair advantage of another's necessities or distress. Civ. Code Dak. § 886. Undue influence at elections is where any one interferes with the free exercise of a voter's franchise, by violence, intimidation, or otherwise. It is a misdemeanor. 1 Russ. Crimes, 321; Steph. Crim. Dig. 79.
-- Black's Law Dictionary
Legal Definition
Whatever destroys free agency and constrains the person to do what is against his will and what he would not do if left to himself, whether the control be by force, threats, importunity or any other sort of mental or physical coercion. See 123 Mo. 1, 45 Am. St. Rep. 505, 25 S. W. 359.
-- Ballentine's Law Dictionary