Semi-formal attire is an outfit that is dressier than what you'd wear to an office but not as dressy as a formal evening gown or tuxedo. Here is more info: https://www.brides.com/semi-formal-wedding-attire-4800687

Women have quite a few options with semi-formal attire. This may include a pantsuit in a dressy fabric, such as silk, cashmere, or satin. A dress or dressy suit with a dressy top, with heels, strappy sandals, or dressy flats may also be worn. Sparkling gemstones, pearls, and dressy fashion jewelry are all appropriate. However, be careful not to overdo it, or you'll look more like you're in costume than semi-formal wear.

Men should wear a conservative dark suit or dress pants with a dress shirt to a semi-formal event. A vest that matches the suit is optional. In most cases, a tie should be worn, but in some cases, it isn’t necessary. If in doubt, wear a tie. You can always take it off later. A belt that matches dress shoes and dark socks are essential. Subtle jewelry is appropriate.

Shorts, Flannels, Jeans, Work Pants, Camouflage, White Dresses