The Collaborative International Dictionary
Undue \Un*due"\, a.
Not due; not yet owing; as, an undue debt, note, or bond.
Not right; not lawful or legal; improper; as, an undue proceeding.
--Bacon.-
Not agreeable to a rule or standard, or to duty; disproportioned; excessive; immoderate; inordinate; as, an undue attachment to forms; an undue rigor in the execution of law.
Undue influence (Law), any improper or wrongful constraint, machination, or urgency of persuasion, by which one's will is overcome and he is induced to do or forbear an act which he would not do, or would do, if left to act freely.
--Abbott.
Wikipedia
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.