"In the field of Deaf Studies, the use of an upper case ‘D’ in the word ‘Deaf’ denotes membership of a Deaf community and use of an indigenous signed language as a primary or preferred language.
Use of the lower case ‘d’ in the word ‘deaf’ refers to people who have a medically determined hearing loss, but who may not consider themselves to be a member of the Deaf community, and who may not use an indigenous signed language.
A typical example of a ‘deaf’ person is an adult with an acquired hearing loss.
A typical example of a ‘Deaf’ person is a prelingually deaf child who, through use of an indigenous signed language, shared linguistic and cultural values with other signed language users."
Signed Languages in Education in Europe – a preliminary exploration
Lorraine LEESON, Centre for Deaf Studies, School of Linguistic, Speech and Communication Science. Trinity College Dublin, 2006