both

severity by tester priority by developerseverity defines how seriously the defect is impacting on other functionality functionality of the application, is giving by the testing team at the time of reporting defects.priority defines how soon the development team need to fix the defect

Both

Normaly Priority is decide by Tester & Severity decide by Developers / any Project Leading person.But if the tester have enough experience then he/she can also decide the Priority of the Bug.