How to assign defect priority and severity?

Defects are given a priority and severity level. Such classification is absolutely needed as the development team can not resolve all defects simultaneously so we need to indicate how soon we want to get the defect fixed, and how big the impact on the functionality of the application under test is.Here are some classification levels:

Defect priority

High Priority: Fix the defect immediately.A core functionality fails or test execution is completely blocked.

Medium Priority: Fix the defect soon.An important functionality fails but we don't need to test it right away and we have a workaround.

Low Priority: Don't fix this defect before the high and medium defects are fixed.

Defect priority indicates the impact on the test team or test planning. If the defect blocks or greatly slows down test execution, we might want to select the highest grade for the defect priority.

Defect severity

Critical: A core functionality returns completely invalid results or doesn't work at all.

Major: This defect has impact on basic functionality.

Moderate: There is impact on the business, but only in a very few cases.

Minor: The impact on the business is minor. Any user interface defect not complicating the functionality often gets this severity grade.

Defect severity indicates the impact on the business of the client. If important functionality is blocked or if that functionality functions incorrectly,then we mostly selects the highest defect severity.
Assigning a defect priority and severity is always subjective as we measure the impact from our point of view.