Depending on who you ask, severity, site event, serious event, or something else. Rachel probably picked up the nomenclature at FB, and even there the origin of the term is kind of lost in the mists of time.

SEVs are severe on-call issues at FB. They look like SEV3, SEV2, SEV1, etc.

Other places may use similar terminology but OP is at FB.

Some places use eg P1 P2 P3 instead of SEV1 SEV2 SEV3

the classification can be in terms of impact to the business. E.g. "P1" could be reserved for issues severe enough to prevent the business from functioning as a business. (E.g. the bank that cannot process customer transactions, the cdn that cannot distribute content)

P3 might mean some features of a service are broken and 1% of your customers are pissed but there is a workaround available or the features aren't really critical

'Severity'. Normally stands for a severe issue on going. They even have levels. SEV3 -> SEV2 -> SEV1 goes from low to highest form of severity.

If memory serves, it's Facebook's incident response process. I don't know that it stands for anythign specific.

"SEVerity" level I think...