Deafness refers to a partial or complete hearing loss. Levels of impairment vary from a mild but important loss of sensitivity to a total loss of hearing. Many sensorineural hearing losses result from a genetic predisposition.

The incidence of significant permanent congenital hearing loss is about 1 in 1,000 live births. This almost doubles by the age of 10 years because of acquired hearing loss from meningitis, mumps, measles, trauma and other causes. Age-related hearing loss affects 30-35% of the population between the ages of 65 and 75 years and 40% of the population over the age of 75. A substantial number of hearing impairments are caused by environmental factors such as noise, toxins and drugs. Hearing is extremely important for the development of spoken language. Those with pre-lingual profound hearing loss will not have access to speech sounds and will not develop oral language, whilst those with a severe pre-lingual hearing loss will not develop normal speech. Moderate hearing loss may affect speech and (oral) language development.