Though both terms "deafness" and "hearing impairment" refer to a degree of hearing loss, they are associated with various definitions depending on context and language. When in doubt, it is best to ask for clarity from the person in question. In this handbook, hearing impairment will refer to a broad panel of hearing losses, including deafness. The latter will then refer to a level of profound or complete hearing loss that keeps one from understanding speech, with or without amplification. A hearing impairment can be due to a sensorineural hearing loss (damaged inner ear), a conductive hearing loss (altered sound transmission from the external or middle ear to the inner ear), or a mixed hearing loss (combination of sensorineural hearing loss and conductive hearing loss). Regardless of the impairment's cause, several levels of hearing losses are categorized: mild (loss of 25 to 40 dB), moderate (loss of 40 to 60 dB), severe (loss of 60 to 80 dB), and profound (loss of more than 80 dB).

     To make up for hearing impairment, one can wear hearing aids or a cochlear implant. In addition, some more informal techniques such as lip-reading and guessing using context cues can help discern the general topic of a discussion. However, these solutions cannot provide full compensation for one's hearing loss. Indeed, even when one's hearing aids are well functioning, they do not bring their hearing level up to an average hearing person's. Moreover, lip-reading mostly works in expected situations where one's interlocutor talks about a known topic and if the keywords are simple. Thus, even when mild or moderate and partially compensated by an amplification system, a hearing loss can make social interactions, lectures, and many other parts of one's everyday life very challenging and taxing. Note that a hearing loss is not selective and occurs regardless of a situation's level of complexity: one might not be able to hear or understand even the most straightforward words during a discussion. This can lead to confused reactions or unexpected questions as their answers seem evident to hearing people. In general, only a written document can assure that the information will be understood completely. In some cases, the person with a hearing impairment may not be aware that they miss out on a piece of information.

     A related condition is tinnitus. Tinnitus translates to having ringing or other noises in one or both ears. Those noises are not caused by external sound and can usually only be heard by the person in question. The noises can be very diverse, as can be their volume. For some, the noises come and go, whereas, for others, they are constantly present. Tinnitus can alter one's ability to concentrate, fall asleep, hear external sounds and is associated with anxiety or depression.