na-adjective
Describes events, clothing, language, or behavior that is official, ceremonial, or follows strict conventions.
今夜はフォーマルなパーティーだから、スーツを着て行くよ。
Tonight's party is formal, so I'll go in a suit.
ビジネスメールではフォーマルな表現を使う
You should use formal expressions in business emails.