The word deaf can have very different meanings depending on the background of the person speaking or the context in which the word is used. Medically, the term can be used to mean having profound hearing loss, a physiological condition causing an inability to receive or process aural stimulation (i.e., sound). Culturally, it can be used in reference to individuals who see themselves as part of Deaf culture. The word “deaf” used in a cultural sense is almost always capitalized (Deaf), while in a medical sense is almost always lower case (deaf).

In the medical view, the global deaf population is roughly estimated to be 0.1% of the total population (1 in 1000). The figure is likely to be higher in developing countries than developed countries due to restricted access to health care, and, in some cultures, due to the high rate of intrafamilial marriages, as in some of the Beduin tribes in the southern part of Israel. Worldwide, at least 5% of the population (1 in 20) is estimated to have less than average hearing. The great majority of people with less than average hearing are elderly or developed hearing loss after leaving school. [http://gri.gallaudet.edu/Demographics/factsheet.html

A minority of deaf people are part of Deaf culture. They are mostly either individuals who were born deaf (Pre-lingual) or became deaf at an early age (Peri-lingual or Post-lingual) and who have a "severe or profound hearing loss;" or are children of deaf parents. Members of Deaf culture use sign language as their primary language and often emphatically see themselves as not disabled, but rather as members of a cultural or language minority. Members of this group use Deaf as a label of cultural identity much more than as an expression of hearing status.