Shoulder-length describes hair that comes down to the shoulders and then stops — not a short crop, not hair down the back. You will almost always hear it with hair: shoulder-length hair. It is a calm, ordinary dressing-room word, not slang and not technical. Though actually, people use it loosely. Hair a little above or a little below the shoulder still gets the label, because nobody brings a ruler. You can say her hair is shoulder-length, or she has shoulder-length hair. Other length-words copy the pattern, but this one stays with hair. It does not mean “how long your shoulder is,” as if you were measuring the bone.