An unambiguous, observable, and if at all possible, measurable standard of acceptance for a product/process characteristic. The purpose of an operational definition is to have a common set of rules for evaluating a characteristic that everyone can follow independently, without any opportunity for misinterpretation. The process for creating operational definitions involves three elements: a criterion, validation test and decision.
The criterion establishes the objective for the key quality characteristic against which the test results can be evaluated. The test provides a specific procedure for measuring the characteristic. Finally, the decision is a determination as to whether or not the characteristic meets the criterion.
Operational definitions provide a foundation for determining whether or not actions need to be taken to improve process performance and should be created prior to the collection of any data.
Operational Definitions from Balanced Scorecard.org: - http://www.balancedscorecard.org/files/opdef.pdf