An exostosis (plural: exostoses) is the formation of new bone on the surface of a bone, because of excess calcium forming. Exostoses can cause chronic pain ranging from mild to debilitatingly severe, depending on the shape, size, and location of the lesion. It is most commonly found in places like the ribs, where small bone growths form, but sometimes larger growths can grow on places like the ankles, knees, shoulders, elbows and hips. Very rarely are they on the skull.
They normally form on the joints of bones, and can grow upwards. For example, if an extra bone formed on the ankle, it might grow up to the shin.
When used in the phrases "Cartilaginous exostosis" or "Osteocartilaginous exostosis", it is considered synonymous with Osteochondroma. Some sources consider the terms to mean the same thing even without qualifiers, but this interpretation is not universal.