A halter is a device that's put on animals to restrain, lead, or train them. Another type of halter is a sleeveless top that ties around the neck.
Your dog might wear a halter that keeps her from pulling you down the street, and a farmer might put a halter on a cow before leading her into a milking stall. Another halter is a piece of clothing usually worn by a woman, a shirt that ties behind the neck and leaves the shoulders and arms bare. Originally, halter referred just to the tie that secures the top, but it came to mean the entire garment.