In Western clothing semi-formal is a grouping of dress codes, indicating the sort of clothes worn to events with a level of protocol between informal (e.g. lounge suit) and formal.
In the day time (before six o'clock), the semi-formal code requires for men a black tail-less coat with formal (striped or checked) trousers. This combination is a less formal version of morning dress - which features a longer, cutaway morning coat. This semi-formal ensemble is sometimes referred to as a "stroller" in North America. For evening wear, the corresponding code is black tie (often simply tuxedo in American English);
The term stroller is only used in the US and is unknown in the UK, where, though the garment itself saw a limited period of popularity, was simply called black lounge. Because black was then reserved for formalwear, it was unknown as a colour for lounge suits, so the term was unambiguous. In the UK this mode of dress is now extremely unusual, though some Masonic Lodges which meet during the day rather than (as is more common) in the evening do continue to specify it as their dress code. It also still is worn within the legal profession, especially by barristers, indeed the striped trousers are in some circles referred to as "barrister trousers". In German, a stroller is called a Stresemann, after the Weimar German chancellor Gustav Stresemann. Stresemann, like other German politicians of his age, wore morning dress or a frock coat in the Reichstag or when making public appearances. However, Stresemann found the long coat impractical for daily work in the Chancellery. To avoid having to change completely, he began to wear the prototype of this jacket at his office, while switching to a morning coat when engaged on more formal business. The style quickly caught on as a more practical variation on morning dress. In Japan, it is known as a "director's suit", from the term inside director.