Tinnitus is a phantom sound. It is a perception of sound originating within the body not actually related to any outside source.
Subjective tinnitus is much more common than Objective tinnitus.
There are many possible causes for tinnitus. Some are not serious. Tinnitus can also be a symptom of a head and neck aneurysm or acoustic neuroma, either of which can be life threatening. These problems often involve a loss of hearing. The treatment will be quite different in each case.