A tailcoat is a knee-length coat characterised by a rear section of the skirt (known as the tails), with the front of the skirt cut away.
The tailcoat shares its historical origins in clothes cut for convenient horse-riding in the Early Modern era. From the 18th century, however, tailcoats evolved into general forms of day and evening formal wear, in parallel to how the lounge suit succeeded the frock coat (19th century) and the justacorps (18th century).
Thus, in 21st-century Western dress codes for men, mainly two types of tailcoats have survived:
Dress coat, an evening wear item with a squarely cut-away front, worn for formal white tie
Morning coat (or cutaway in American English), a day-wear item with a gradually tapered front cut away, worn for formal morning dress
In colloquial language without further specification, "tailcoat" typically designates the former, that is the evening (1) dress coat for white tie.
In equestrianism, a variant called a shadbelly is still worn in certain disciplines in its eighteenth-century role as daytime formalwear. It is basically a form of dress coat which is closer in cut to the early nineteenth-century style worn by Beau Brummel than to the modern version worn with evening formal dress. The male version of the shadbelly is often called a "weaselbelly".
Court uniform and dress in the United Kingdom
This is a type of dress coat traditionally worn with court dress, until the mid-twentieth century. It was made of black velvet and traditionally worn at court, levées, and evening state parties by those who did not wear uniforms. A version made of black barathea was also worn as diplomatic dress.
It was single breasted with a stand-up collar, with plain gauntlet cuffs, and two three-pointed flap pockets on the waist seam. It had six metal buttons at the front, and two decorative buttons at the back. The body of the coat was lined with black silk, and skirts with white silk. It was worn with breeches, black silk hose, white bow tie, white gloves, and court shoes (pumps) with steel buckles.