Tinnitus or "ringing in the ears" is the perception of sound in the brain that does not exist in the external world. Unlike a hallucination, tinnitus has an organic cause and merely persists as a constant noise rather than being perceived as meaningful sound. Most people experience it at some time in their lives. However, in some individuals, it can interfere with sleep or other activities and requires medical intervention.
Ironically, the most common cause of tinnitus is hearing loss, usually from damage to the cochlea. However, it can also be caused by aspirin overdose, Meniere's disease, brain tumor, abnormal blood flow, or even misalignment of the bones of the jaw.