Generally we come across the two common terms severity and priority while posting bug
severity is the one which is describe as the bug which has m0re impact on the application. It may be application crash or unable to proceed some functions in the application .
The issue which causes the application not to proceed further is called severity
we know very well about the word priority which means giving importance. similarly in testing priority means which bug has to be fixed first (E.g. if u posting 10 bugs means based on the priority level the order of bug will be fixed.)
The bug which has higher priority wil be fixed first, same case for severity too
the level of priority and severity can be measured as
priority --- p1,p2,p3 ( p1 is the highest priority level)
severity -- s1,s2,s3 ( s1 is the highest severity level)
Severity is Technical, means it will explain how adversely it is effecting from technical perspective where as Priority is bussiness means what is the business impact of that issue.
More over Priority is relative means it will vary based on timelines you have and the stage of testing at which you are..where as Severity is Absolute..means it is fixed irrespective of the stage of the project.
Severity is the impact of the funtionality of the application,and priority is related to business perspective and how quicly the defect is removed from the application mainlty decided by the developer.