The doctrine allowing recovery on a promise made without consideration when the reliance on the promise was reasonable, and the promisee relied to his or her detriment. See e.g. Cohen v. Cowles Media Co. 501 US 663 (1991).
Definition from Nolo’s Plain-English Law Dictionary
A legal principle that prevents a person who made a promise from reneging when someone else has reasonably relied on the promise and will suffer a loss if the promise is broken. (See also: estoppel)