Morning suit

Morning dress is the daytime formal dress code, consisting chiefly of, for men, a morning coat, waistcoat, and striped trousers, and an appropriate dress for women. Men may also wear a popular variant where all parts (morning coat, waistcoat and trousers) are the same colour and material, often grey and usually called 'morning grey' to distinguish it; this is only properly appropriate to weddings and races and is known as a morning suit. The semi-formal counterpart of this code is the stroller. Morning dress is now rarely worn as anything other than formal wear, as a form of civic dress, e.g., by provincial mayors (as an alternative to court dress), but more generally only for weddings, some official civic, governmental or royal functions, 'social season' events, e.g., races such as Royal Ascot (where it is obligatory in the Royal Enclosure) and at Epsom in the Queen's Stand on Derby Day, formal lunches (especially those in the City of London institutions, notably of the livery companies and guilds) and as uniform at some of Britain's most traditional schools such as Eton. It will also be seen sometimes worn at services in St Paul's Cathedral, London and St Giles' Cathedral, Edinburgh.

The name originated from the practice of gentlemen in the nineteenth century riding a horse in the morning with a cutaway front, single breasted morning coat. The modern twentieth century morning dress was originally a more casual form of half dress, but as the nineteenth century progressed it gradually became acceptable to wear it in more formal situations instead of a frock coat. In the Edwardian era it took over in popularity from the frock coat as the standard daytime form of men's full dress. When it was regarded as a more casual coat, it was common to see it made with step collars (notched lapels in American English), but as it took over from the frock coat in formality it began to be made with the more formal pointed lapels (peaked lapels in American English).

...
Wikipedia