An illusory promise is a statement which cannot be legally enforced because it is too vague or is impossible to fulfill. Such promises may be worded like contracts, but they are not contracts and cannot be enforced as such. In other words, when someone makes an illusory promise, it creates no binding liability; that person does not need to follow through.
There are a number of reasons why an assurance made by someone may not be legally valid. One example could be a situation in which the fulfillment of the statement depends solely on the whim of the person making it. This can be seen in the case of conditional offers which depend on whether or not the person making the offer wants to make it good. Likewise, people may make promises which simply cannot be realistically fulfilled, such as agreeing to buy as much of something as someone is willing to sell.
Lack of mutuality can also be grounds for considering a statement an illusory promise. If only one person in an agreement is receiving consideration in the form of a service, money, a pledge to do something, or a promise not to do something, this is not considered a binding legal contract. For example, if someone says "I will bake you a pie tomorrow," this is an illusory promise, because the subject of the pledge is not being asked for anything in return and the baker is receiving no consideration for the promised activity.
There may be cases in which an illusory promise is taken to court by someone who believes that the promise should have been legally valid. In these cases, the plaintiff is suing for breach of contract and may request that damages be awarded. The court will review the situation, and if it believes that a statement was not legally enforceable, it will rule in favor of the defendant.
It is important to be aware that courts tend to lean more towards enforcing contracts than declaring them invalid. Statements which are vague and poorly defined are usually considered illusory promises. If a court believes that there is a valid interpretation of such a statement, however, it may rule that it is legally binding and the person who made the promise will be obliged to make good on it. For this reason, it is advisable to be very careful about the wording of promises which could potentially taken as contracts.