Haley has an interesting take on the definition of business rule.
Business rule :is a statement that defines or constrains some aspect of the business. A business rule asserts business structure to control or influence the behavior of the business, i.e. a process or procedure.
That invaluable tool, the Wikipedia, has the cleanest and concise definition of the term 'business rule' I've encountered. The Wikipedia defines business rule as:
From humble beginnings in the 1990s, business rules technology evolved into commercial-grade platforms capable of driving big and complex systems for Fortune 500 companies. Some technical infrastructures cost millions of dollars to implement and deploy. Business rules technology has become essential to the functioning of core business processes.