Any noise you hear that is not a real sound that others can hear is tinnitus. The actual experience for people with the condition varies, for some it's a hiss, for some it's a tone, for me it's a really loud, multi-tonal, warbling sound between 11khz and 15khz. If anyone has tinnitus and wants to know what frequency it is that you your brain is perceiving just go online and find a tone generator and start increasing the frequency until the sound from the speakers suddenly disappears. That's the frequency of your tinnitus.
e: btw tinnitus is considered hearing damage.
I've always had tinnitus, but it used to be that I could only hear it in absolute silence, but it was a medication that triggered mine to go from barely there to screaming banshees in my ears 24x7x365. It sucks to know that I will never truly experience silence again, but my brain does tune it out most of the time. But it's mostly noticeable at night. Mostly.