When it comes to the world of horses, the term "mare" refers to an adult female equine, whether it's a horse or another member of the equine family. Typically, a mare is considered to be a female horse that's three years old or older, while a younger female horse is often called a "filly." However, in the realm of Thoroughbred horse racing, a mare is specifically defined as a female horse that's reached the age of four or older. So, there you have it – a quick glimpse into the world of mares in the equine universe!