1. A disorder of the middle ear that results in progressive deafness: Otosclerosis normally affects both ears and it usually occurs early in adult life.
2. The abnormal formation of new bone in the middle ear that gradually immobilizes the stapes (stirrup bone) and prevents it from vibrating in response to sounds, causing progressive loss of hearing: The deafness caused byotosclerosis may be partially overcome with a hearing aid, but surgery is eventually needed and this involves replacing the stapes bone with a synthetic substitute (stapedectomy).