Semi formal could best be described as a well-kept look. Men can dress in nice slacks and a button-down shirt, but not necessarily a sport jacket or full suit. A tie or bowtie should also be worn in consideration of the "formal" attire.
A nice suit that fits well paired with a tie of coordinated color, or dress pants with a blazer or suit jacket and tie would be considered semi formal.
Either khaki or black pants with a button down shirt and sports coat. Also acceptable is a suit with dress shirt and tie. Never acceptable is a polo-style shirt with a sport coat.
For men, formal attire usually refers to a tuxedo. Semi-formal attire is usually a suit, but in some cases may only be nice slacks with a shirt and tie.
