in case of emergency; should danger arise
A formal, somewhat literary yojijukugo used to express preparedness for a sudden crisis or emergency. Often appears in written or set-phrase contexts.
In case of emergency, please act according to this manual.
He always keeps an emergency kit ready, just in case.
A more common, everyday term for 'emergency' or 'urgent situation', while 一旦緩急 is a formal, set phrase emphasizing preparedness for a sudden crisis.
Means 'by any chance' or 'in the unlikely event', often used in conditional clauses. 一旦緩急 carries a stronger nuance of a dangerous emergency.
A yojijukugo combining 一旦 (once, in case) and 緩急 (slowness and urgency, i.e., emergency). The exact historical derivation is uncertain, but it is conventionally used to mean 'in case of emergency'.