Both Severity and Priority are attributes of a defect and should be provided in the bug report. This information is used to determine how quickly a bug should be fixed.
Priority by the English definition is used in the comparison of two things or conditions, where one has to be given more importance than the other(s) and has to be tackled with/resolved first before proceeding to the next one(s).
Therefore, in the context of defects, the priority of a defect would indicate the urgency with which it would need to be fixed and deployed to live servers
Severity by the English definition is used to describe the gravity of an undesirable occurrence.
Hence when it comes to bugs, the severity of a bug would indicate the effect it has on the system in terms of its impact. Usually the severity is defined in terms of financial loss, damage to environment, company’s reputation and impact on end user.
When a defect is of high severity, most likely it will also have a high priority. Likewise, a low severity defect will normally have a low priority as well.
Below are some examples for more understanding
High Severity – High Priority bug
This is when major path through the application is broken, for example, on an eCommerce website, every customer gets error message on the booking form and cannot place orders, or the product page throws a Error 500 response.
High Severity – Low Priority bug
This happens when the bug causes major problems, but it only happens in very rare conditions or situations, for example, customers who use very old browsers cannot continue with their purchase of a product. Because the number of customers with very old browsers is very low, it is not a high priority to fix the issue.
High Priority – Low Severity bug
This could happen when, for example, the logo or name of the company is not displayed on the website. It is important to fix the issue as soon as possible, although it may not cause a lot of damage.
Low Priority – Low Severity bug