What is a Unique Selling Proposition?

A Unique Selling Proposition is typically a short statement, tag line, or slogan ...

... that succinctly describes what distinct benefits or value a business, product, or service delivers to customers.

Examples:
M&Ms ... "Melts in your mouth, not in your hand."
Head & Shoulders ... "You get rid of dandruff."
FedEx ... "When your package absolutely, positively has to get there overnight."
Raytheon Missile Systems ... "Customer success is our mission."