What is the doctrine of promissory estoppel, and when is it used?

Promissory estoppel is a doctrine used to enforce noncontractual promises where there has been justifiable reliance on the promise and justice requires the enforcement of the promise. It is used to avoid injustice when the promisor reasonably expected that the promisee, in reliance on the promise, would be induced to take definite and substantial action or to forbear, and the promisee does take such action or does forbear.