Definition
The FMEA score for how serious the consequence of a failure is; it describes the consequence only, independent of how often the failure happens or whether it can be detected.
In detail
Severity is the least negotiable of the three scores: it follows from the consequence, and actions can only move occurrence and detection. The only way to move severity is a redesign that removes the consequence itself. The source material calls for special attention when S = 9 or 10; by default ABS puts items at or above a threshold on a mandatory action list (the threshold is customer-calibrated), so ranking by RPN alone is not enough — but that action list is an ABS policy, not a universal hard rule from the textbook. Two rating scales coexist, 1 to 10 and 1 to 5, so any score quoted must state which scale it came from; otherwise two FMEAs cannot be compared.
Example in practice
A seal failure can leak fluid and injure someone, so the team rates severity 9. Adding an in-line leak test moves detection from 8 to 3, but severity stays 9, so the item remains in the action plan until a one-piece moulded design removes the leak path altogether.