Bug Severity vs Priority

Severity - refers to the effect the bug has on the functionality of the application

Priority - refers to how soon the bug has to be fixed.

If we term the priorities as P1, P2 , P3 and P4, where P1 is the High priority issue and P4 is the low priority,

we can define the Severity as follows.

P1 - Complete loss of service(of the application)-
showstopper

P2 - Severe loss of service- business logic issues

P3 - Minimal loss of service - which will not effect the core business logic but still must be resolved

P4 - Needs improvement - all the UI issues.

Most of the times, a tester assigns the severity and Leads will assign the priority