Brand promise
The single thing a brand commits to deliver every time someone buys or uses it, stated so that customers can check it. A promise is useful when the business is built to keep it and staff can act on it; it becomes empty when it is a slogan the operation cannot back. Most buyers will not remember the wording, so the promise matters more as a guide for what the company does than as a line in ads.
Where it's used
Used in brand strategy, service design and internal brand training to say what customers can always count on. Good when the promise is specific enough that a customer could tell if it was broken.
Example
Overnight, built into the network
FedExIn 1978 FedEx advertised When it absolutely, positively has to be there overnight. The line worked because the company had built its operations around it: planes flying parcels into a central hub in Memphis at night and out again before morning. Customers could test the promise with every delivery. A promise is only as strong as the system behind it.