Tinnitus is simply the perception of sound when no actual sound is present.

There are many different sub-categories of tinnitus, including the type you describe that results from permanent hearing damage.

However, there are other causes of tinnitus that can come and go, or be worsened by different conditions. Some people have tinnitus that changes volume by clenching their jaw, for example.

Classic subjective tinnitus is thought to be similar to phantom limb syndrome, where the patient experiences sensory inputs that are not actually present.

There is even a category called Objective Tinnitus, which is tinnitus that an external observer can hear (with the right tools). This is usually caused by strange blood flow patterns or muscle spasms.