Plain English Meaning
A risk level meaning 'this could really hurt someone or break something important, but it probably wouldn't kill anyone or destroy the aircraft.' It is one step below the worst category.
Full Definition
In the FAA risk assessment matrix, a severity level describing a hazard whose potential outcome is a major injury, major property damage, or a significant reduction in the ability to complete the flight safely, but stops short of catastrophic (loss of life or aircraft).
Context Anchor
Seen in FAA risk assessment tools and PAVE checklist discussions when deciding whether a flight, lesson, or training activity is safe enough to continue.
Derivation
From Latin serius, meaning 'weighty' or 'grave.' In risk language, it keeps that : a Serious outcome is enough to a great deal, even if it is not the worst possible.