noun
very serious matter; momentous event
Used for matters of grave concern or events with significant consequences. Often appears in formal or news contexts.
この問題は国家の重大事だ。
This issue is a matter of grave national concern.
彼の辞任は会社にとって重大事だった。
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).