I’d wear a suit and tie. If you feel overdressed then take off the tie and/or take off the suit jacket More on reddit.com

I have a team banquet for my college cross country team (men's team, women's team, coaches, superintendents, and some parents will be there) and my coach said that the attire is semi-formal. However, he also said "You don't need to wear a full suit, just a simple sweater with a tie underneath... More on styleforum.net

It's a term that's overused enough that you have to figure it out from the context of the situation or event. In this context, it almost certainly means "dress up, but not like a suit, man". Assuming you're young, you'll probably be fine with with a button-up tucked into chinos and some dress shoes. More on reddit.com

To me, semi formal means a man wears a suit and tie, and a woman wears a cocktail type dress that need not be especially flashy. Dress shoes, although the woman can obviously wear low heels or flats if that's what she is comfortable in. Again, they need not be super fancy but just not running shoes or something of that sort. Exception would be if there are foot or mobility problems involved, in which case you wear the footwear that meets your needs. More on reddit.com