The term 'collar piece' is a type of clothing accessory that is used to fasten the collar of a shirt or blouse. It is usually made of metal and is often referred to as a 'collar STAY'. The crossword solution for this term is 'STAY', which is a four-letter word meaning to remain in the same place or position. This is an appropriate description of the function of a collar piece, as it helps to keep the collar of a shirt or blouse in place.