Semi-formal is the most confusing of all the dress codes. Semi-formal means not a suit and tie but it doesn't mean jeans. Many people come underdressed when they come to a semi-formal event. Semi-formal is slightly more relaxed than a formal event but not relaxed like a house party where you can wear jeans. You can wear a tailored fitted sports coat paired with a nice, fitted pair of dress pants that matches your coat and jacket. Jeans are always considered casual only nothing less nothing more.