Semi-formal in the context of "Morning dress"

The black lounge suit (UK), stroller (U.S.), or Stresemann (Continental Europe), is a men's day attire semi-formal intermediate of a formal morning dress and an informal lounge suit; comprising grey striped or checked formal trousers, but distinguished by a conventional-length lounge jacket, single- or double-breasted in black, midnight blue or grey. This makes it largely identical to the formal morning dress from which it is derived, only having exchanged the morning coat with a suit jacket, yet with equivalent options otherwise, such as necktie or bowtie for neckwear, a waistcoat (typically black, grey, or buff), French cuffs dress shirt of optional collar type, and black dress shoes or dress boots. The correct hat would be a semi-formal homburg, bowler, or boater hat. Just as morning dress is considered the formal daytime equivalent of formal evening attire dress coat i e. white tie, so the stroller is considered the semi-formal daytime equivalent of the semi-formal evening attire dinner jacket, i.e. black tie (also called tuxedo). Unlike other dress codes, there is no clear equivalent for women, though typical morning dress and cocktail dress have both been identified as alternatives.

Wearing a black lounge suit the traditional way with formal trousers largely fell out of use following the counterculture of the 1960s, although its practice has still been observed occasionally ever since.