The term semi-formal is also used in formal methods (computer science) to mean not fully semantically formalized.

In Western clothing, Semi-formal is a grouping of dress codes, indicating the sort of clothes worn to events with a level of formality between informal (i.e. lounge suit) and formal.^[1][2] Historically there were both day and evening semi-formal dress codes, but this category is now seen almost exclusively in the context of evening wear. In the day time (before six o'clock) the semi-formal code is a black tail-less coat with formal (striped or checked) trousers (this combination is called stroller in America). For evening wear, the corresponding code is traditionally black tie (tuxedo in American English), but the interpretation and description of dress codes in different social settings and continents now varies, so events that would previously have required strict black tie may now not expect this.^[1]

Terms like 'informal' or 'semi-formal' are used to describe a classification of dress, so would not be directly given on an invitation; instead, the direction for the guest will normally be 'lounge suit' or 'black tie' (tuxedo) where appropriate, which removes some of the ambiguity.

Women traditionally wear either an evening gown or a cocktail dress, depending on the time of day.

Semi-formal dress codes

Eveningwear

Black tie (Tuxedo)

Morningwear

Stroller

References