Breast-collar is a term used to describe a part of the harness worn by horses to attach the cart or carriage. While the word breast-collar is commonly used, there are other synonyms that can be used interchangeably to describe the same object. These synonyms include "breastplate," "chest plate," "breast strap," and "martingale." Each term refers to the same part of the harness and is commonly used by horse enthusiasts. It's important to note that some horse riders may prefer one term over the others, but they all serve the same function - to secure the harness and keep the horse safe.