Noun
A formal evening jacket with an extended back panel; a dress coat.
Any coat with similar tails.
Source: en.wiktionary.orgWhether you like the look or not, that tailcoat is a tough shell, a suit of armour. The posh boy is a hardy species. Laura Wade
Thus it was worn with the standard accompaniments for the evening tailcoat at the time: matching trousers, white or black waistcoat, white bow tie, white detachable wing-collar formal shirt and black formal shoes. Source: Internet
The wearing of shadbelly or other tailcoat jackets is not compulsory in the dressage phase. Source: Internet