How to use "Severity" and "Priority"

A ticket contains the two (quite similar sounding) fields "Severity" and "Priority".

Severity

"Severity" holds the values "critical", "high", "medium", "low" and "trivial".

With this field the creator/reporter of a ticket tells how "severe" this bug is (from the reporter's point of view).

"Critical" should only be used if a bug results in data loss or makes the application unusable.

Priority

"Priority" holds the values "blocker", "critical", "major", "minor" and "trivial".

This field is used by the project management to prioritize the existing tickets. Higher ranked tickets are more likely to go into the next release. When planning a release one will typically try to ensure that all tickets with priority "blocker", "critical" or "major" are fixed for the scheduled milestone.

A "normal" user should leave this field unchanged.