A vestment is a garment worn at special ceremonies by a clergy member. For example, a priest would wear a vestment in church, but out in the community, he would wear a shirt and pants.
You know that a vest is a piece of clothing — a sleeveless shirt or sweater. Vest can also be a verb that describes putting on clothing, typically garments related to a religious ceremony. Both vest and vestment come from the Latin word vestimentum, meaning "clothing, clothes." Vestments include ankle-length robes called cassocks and strips of fabric that go around the neck, which are called stoles.