Definition: Hearing loss is the decreased ability or inability to hear sound. Hearing loss can affect one or both ears. Hearing loss can be caused by a mechanical problem, such as damage to the small bones in the ear, or a problem with sending a signal to the brain indicating that a sound has been heard. Hearing loss may be present at birth or develop over the course of life and may be caused by a number of factors: genetics, infectious diseases, trauma, certain medications, long-term exposure to loud noises, and aging.