The term "semi-formal" is used to depict the category of attire that's the dress code for a particular event. Semi-formal dress falls between casual and black-tie or formal. Semi-formal dress may also slightly vary, according to the time of the event. Semi-formal dress codes can be somewhat confusing; as people may have different ideas about what's appropriate. However, there is a general rule of etiquette for such occasions, which can help you fill in the grey areas.
Daytime and Afternoon Semi-Formal
Semi-formal attire for the morning or afternoon typically means that a man wears a suit and a woman can wear a nice suit or a dress that's not full length.
Semi-formal events in the evening (such as a wedding), would call for a man to wear a suit that is dark in colour and a woman to wear a cocktail dress or long evening gown.
Semi-formal attire for men calls for a suit (vests are not required but can be worn), a button-down collar shirt and a bow or tie.
Semi-formal attire for women tends to be a bit more flexible because they can choose from dress suits, cocktail dresses or pantsuits.
The fabric type of the clothes should also be considered (especially when it comes to evening semi-formal). Men should avoid all-cotton classic or linen suits and women should avoid business suits that are made of wool.
If you have a semi-formal event to attend and are not sure what to wear, it's a good idea to browse the appropriate categories in different online retail shops to get a better picture.