A benign lesion of bone, probably not a true neoplasm, consisting chiefly of fairly dense, moderately cellular, fibrous connective tissue in which there are small foci of osteogenesis. Most examples of this condition, especially in the maxilla and mandible, probably represent foci of fibrous dysplasia; a few examples of fibrous lesions with foci of osteogenesis, especially in vertebral bodies, may be neoplasms.