adj

• (Canada, US, specifically) Of clothing: having a level of formality between that of informal and formal wear.
  "Suits have largely replaced guayabera shirts as business and semiformal wear for middle and upper-class Panamanian men, made practical by the widespread use of air-conditioning in the cities."
• (Canada, US, specifically) Of an event: having a semiformal dress code.
  "The skirt was way too short, at least shorter than she preferred. It left exposed a good eight inches between her knee and thigh. But what choice did she really have? Casual cropped pants were hardly appropriate attire for a semiformal event."

noun

• (Canada, US) A social event in which one is expected to dress in semiformal wear.
  "Bright-colored flyers decorated battered lockers, telling everyone to join the debate team, try out for the school play, and not to miss the fall semiformal this Friday. I wondered what a "semiformal" consisted of at a public school, not that I'd bothered to ask anyone."