Tinnitus is sound (usually ringing or buzzing) that can is heard in the ears or in the head when there is no external object actually making the noise. It can come and go. It is usually very distracting, and sometimes even distressing, to the person who experiences it. It can block the ability to hear other, real sounds. It can cause sleep disturbances and in some, depression.
Tinnitus is relevant to you in a teaching environment because it may interfere with your deaf student’s ability to hear. That is, his or her hearing may appear to fluctuate and sounds that are heard sometimes are not heard at other times. This can be confusing to you and you may even wonder if your deaf student is “putting it on”. S/he isn’t.