An illusory promise is a promise that is unenforceable due to indefiniteness or lack of mutuality, where only one side is bound to perform. The promise is normally a statement that seems to be an offer or agreement, but is so vague, ambiguous or conditional that it does not actually obligate the promisor to do anything at all. An example of this would be an agreement between a seller and buyer which states that the seller "agrees to sell all of the ice cream he wants to" to the buyer.
An important in contract law that illustrates this concept is Hamer v. Sidway, 27 N.E. 256 (N.Y. 1891). In this case, an uncle promised his nephew that he would pay him $5,000 if the nephew refrained from drinking, using tobacco, swearing, or gambling until the age of 21. The nephew fulfilled his side of the bargain, but the uncle refused to pay, arguing that the promise was merely a gift and not a binding contract. The court held that the promise was not illusory because the nephew had given up a legal right (to engage in those activities) in exchange for the promise of payment.
[Last updated in March of 2023 by the Wex Definitions Team]