Data Validation

Definition:Data validation guarantees to your application that every data value is correct and accurate. You can design data validation into your application with several differing approaches: user interface code, application code, or database constraints.
Example: Simple field and lookup validation is sometimes not enough. Consider a health care claim which has a billed amount of $123.57, but the allowable amount may depend on a year-to-date rolling accumulation that is capped at $1500 (not to exceed the lifetime policy maximum of $100,000). In this situation, data validation extends beyond the immediate data entry screen to one of careful evaluation of how to pay this claim based on the policy limits and both year-to-date and lifetime accruals. This kind of complex multifile data validation is often best handled with procedure-based business rules.     
http://msdn.microsoft.com/en-us/library/aa291820(VS.71).aspx
Picture:
http://blogoscoped.com/files/spreadsheet-data-validation.png