An exostosis ( exostoses) is a benign (non-cancerous) new growth projecting from the surface of a bone, informally known as a bone spur. Exostoses can cause chronic pain ranging from mild to severe, depending on the shape, size, and location of the lesion, though most are asymptomatic and are found by chance on plain x-ray taken for other reasons. Occasionally, an exostosis can be potentially life-threatening.

When used in the phrases "cartilaginous exostosis" or "osteocartilaginous exostosis", the term is considered synonymous