Tinnitus is the perception of sound without external acoustic stimulation. The sound is intermittent, fluctuating or - in most cases - constant, may be perceived as ringing ("tinnire" is the Latin word for "to ring"), roaring, hissing or some other noise, and is usually high-pitched, with up to 40% of people reporting multiple tones. Tinnitus can occur in one ear or both ears.

Most people will experience a transient tinnitus (less than five minutes long) that may not compel them to seek medical evaluation. Fortunately, severe forms are less common. Almost everyone at one time or another has experienced brief periods of mild ringing or other sound in the ear. Some people have more annoying and constant types of tinnitus, which may seriously impact their ability to sleep, relax or to concentrate, or lead to tiredness and nervous-ness. Tinnitus affects both women and men alike (though more so in males), and may occur at all ages, but is most prevalent in 50 to 70 year olds. According to some estimates, 0.3 to 1% of the population may suffer from tinnitus in a way that they are prevented from leading a normal life.

Tinnitus is not a disease by itself, but a symptom common to various hearing disorders, just as pain accompanies many different illnesses. It is most frequently associated with noise-induced hearing loss, age-related hearing loss and Ménière's Disease. Other, less frequent origins include exposure to ototoxic drugs (aminoglycoside antibiotics, high-dose loop diuretics, nonsteroidal anti-inflammatory drugs and certain chemotherapeutic agents), reduced vascular flow (ischemia), autoimmune processes, infectious diseases, conductive hearing loss, otosclerosis, head trauma etc. In the vast majority of cases, tinnitus is associated with hearing loss of known origin, and probably more than two thirds of all cases originate within the cochlea.

Recent scientific findings focus on the glutamatergic synapse between inner hair cells and auditory nerve to explain the genesis of tinnitus within the cochlea. Tinnitus is considered as sort of "epileptic" firing in the auditory nerve, induced by excitotoxicity, a form of neuronal degeneration, which can occur when glutamate is released in large amounts (e.g. after exposure to very loud noise) or when incompletely recycled.