Severity defines how serious a bug is and how badly it affects the functionality. Severity is a predefined field for Bug.
Default severity values are Blocking, Critical, Normal, Small, Enhancement. Blocker severity usually means that the system or functionality is currently unavailable because of this bug. Enhancement is the lowest severity used for minor bugs or typos.
A higher numeric value for Importance means a lower priority.
Administrators can customize the severity values and importance in your account. Navigate to Settings → Business Values → Bugs Severities tab.
To add custom severities, click the add link and fill in the Name and Importance fields.
Also, you can mark a severity default.
Renaming existing severities will have an impact on all the entities where it was chosen in the properties.
If you want to delete any of the listed severities, you will be asked to specify one of the remaining severities where you have to transfer all its related entities.
Severities are defined as global (system-wide) values. It is not possible to define severities per process. Use Custom Fields of type Drop Down List as a replacement instead.