Severity – dangerous, causing fear or anxiety by threatening great harm, a dangerous operation, a grave situation, a grave illness etc
Priority - An item's relative importance, A goal of a person or an organization, Prior or antecedent in time, or preceding something else.
We can see that the two words have completely dissimilar meanings. But still there is a lot of confusion between them?
The priority is the domain of the Client and should be entered by them against each defect showing the importance of the change to them. The severity is the domain of the tester and they should be able of recording this.
The severity to the testers is the impact of the defect on the application and reflects that whether further testing could be done or not. For example, a spelling error would be deemed as a low severity by the tester (Mostly Cosmetic), but if this error occurs in the corporation name or address, this would be classed as high priority (Important) by the business point of view.
An inability to access an infrequently used menu option may be of low priority to the business, but the severity is higher since many of the important scenarios cannot be executed, and are dependent on access to the option. The mistake that we very often make many times is to suppose that the tester is also capable of recording the priority.
It may be possible for the tester to make an educated assessment, testers who have been involved with a particular application for some period of time may be able to do this, but it is necessary to have involvement in the life-cycle of a defect. When a project enters test execution, the spotlight will be on fixing defects of the highest priority.
This means that the application will be released with the minimum amount of priority defects unsolved. Care should be taken by the Project Manager to ensure that severity is not ignored. So a balanced approach is needed, which favors the business priority. At the end of the project the quantity of high severity and high priority defects should have at least been reduced, if not removed.
To summarize:
Priority = Business = Order of Fixing
Severity = Tester = Failure of Application