Editorial
Promissory estoppel
Definition
Where a party to a contract makes a clear and unequivocal promise not to enforce their strict legal rights, and the other party relies on it, the promisor may be prevented from going back on the promise. In English law it operates as a shield and not a sword: it suspends rights rather than creating a new cause of action.
In plain English: If someone promises to let you off part of a debt and you act on it, they may not be able to change their mind — but in England you usually cannot sue on the promise alone.
You did not name a jurisdiction, so this shows the GB definition. The term is defined differently elsewhere — see below.