The Business Rules Group, a non-commercial peer group of IT professionals, defines Business Rules as follows:
A business rule is a statement that defines or constrains some aspect of the business. It is intended to assert business structure or to control or influence the behavior of the business [..]
From the information system [..] perspective, it pertains to the facts that are recorded as data and constraints on changes to the values of those facts. That is, the concern is what data may, or may not, be recorded in the information system.