In database design what is a business rule?

Very good question!

A business rule is just as it states. Lets take a look at a really simple example first; Let say for example a business has a RULE that on its web site when you are filling out a form that the First Name field can only contain 20 characters, that's the rule!

In the database you would apply a constraint (some might do this at program level) that permits the First Name field to only contain 20 character.

Now business rules typically go MUCH deeper than that. Business rules in an application, or program simply represent constraints or processes that you put in place in order to comply with the rules set out by the business. These rules can be corporate rules or simply limitations, or constraints that yield the appropriat outcome as setup in the scope of work (The technical document that tells you what your program needs to do, how it is to look, how end users are to interact with it, ect).

If you need more detail please ask.

Jeff