Definition

The technique used to prioritise the development and execution of tests upon the impact and likelihood of failure of the functionality or aspect being tested.

Comment

Given a limited resources and time to test a product or service, it will never be possible to test every permutation and combination of every element of functionality. If one focuses test development and execution on those aspects of the product or service that are more likely to fail and impact the users, rather than testing those that are sure to work or those that if they failed would have no impact on users.

This is also known as risk-based testing and riskbased testing.