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.

How does this work in combination with age related hearing loss? At some point you will lose high frequency sensitivity in that 11-15Khz range. Would be nice to get some benefit from that, but I assume the tinnitus itself will not go away even if it hangs out at that frequency?

It also means the above experiment will not work since you lose the signal before you reach your tinnitus frequency.

Honestly though, a hiss that you can hardly notice is maybe technically tinnitus, but not a problem at all. Actual tinnitus would drive anyone crazy, I bet.