very serious matter; momentous event
Used for matters of grave concern or events with significant consequences. Often appears in formal or news contexts.
This issue is a matter of grave national concern.
His resignation was a momentous event for the company.
重大 is a na-adjective meaning 'serious' or 'grave', whereas 重大事 is a noun referring to a specific serious matter or event.
Compound of 重大 (serious, grave) and 事 (matter, event).