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:
Declarative
A business rule is a statement of truth about an organization. It is an attempt to describe the operations of an organization, not an attempt to prescribe how an organization should operate. This is why business rules are said to be discovered or observed and not created.
Atomic
A rule is either completely true or completely false; there are no shades of gray. For example a rule for an airline that states passengers may upgrade to first class round-trip tickets if seats are available and they pay the fare increase does not imply that this deal is available for just one leg of the journey.
Distinct, independent constructs
Separate the things that define your business (the rules) from the processes (i.e. strategies and tactics). Don't build complex and cyclical dependencies - simplify and flatten the constructs.
Expressed in natural language
In order to appeal to the broadest audience, it is almost always best to express business rules in a natural language without the use of a lot of technical jargon.
Business, not technology, oriented
For example, a company's business rules should not be foreign to a knowledgeable customer.
Business, not technology, owned
Business rules come from business decisions. These are independent from implementation decisions.