セミフォーマル

na-adjective, noun which may take the genitive case particle 'no', noun

semiformal

Describes attire, events, or settings that are less formal than formal wear but dressier than casual. Common in fashion and dress-code contexts.

• パーティーはセミフォーマルな服ふく装そうでお越こしください。

  Please come to the party in semiformal attire.

• このレストランはセミフォーマルなので、ジャケットを着きていったほうがいい。

  This restaurant is semiformal, so you'd better wear a jacket.