The ridge at the top of a horse's back, right where the neck meets the body and just above the front legs. If you stand beside a horse and run your hand from its neck backwards, the withers is the small hill your hand goes over before the back flattens out. It matters because it is the standard place people measure a horse from: you measure from the ground up to the withers, not up to the head, because a horse moves its head all the time and the withers stays still. The same word is used for dogs and some other four-legged animals, especially in dog shows and breed rules. This word is always plural in form — you say "the withers", never "a wither", even though you are talking about one part of one animal. It is a specialist word: horse people, vets, and dog breeders use it every day, and most other English speakers have never used it in their lives. Many will have met it only in one old phrase from Shakespeare, "my withers are unwrung", meaning "that criticism does not hurt me" — but that phrase is very old-fashioned, and you can safely leave it alone.