A given name (also known as a forename or first name) is the part of a personal name that identifies a person, potentially with a middle name as well, and differentiates that person from the other members of a group (typically a family or clan) who have a common surname. The term given name refers to a name usually bestowed at or close to the time of birth, usually by the parents of the newborn. A Christian name is the first name which is given at baptism, in Christian custom.
In informal situations, given names are often used in a familiar and friendly manner. In more formal situations, a person's surname is more commonly used. The idioms 'on a first-name basis' and 'being on first-name terms' refer to the familiarity inherent in addressing someone by their given name.
By contrast, a surname (also known as a family name, last name, or gentile name) is normally inherited and shared with other members of one's immediate family. Regnal names and religious or monastic names are special given names bestowed upon someone receiving a crown or entering a religious order; such a person then typically becomes known chiefly by that name.
In the last century and well into the present one, grown-up British people, with rare exceptions, addressed each other by their surnames. What we now call first names (then Christian names) were very little used outside the family. Men who became friends would drop the Mr and use their bare surnames as a mark of intimacy: e.g. Holmes and Watson. First names were only generally used for, and among, children. Today we have gone to the other extreme. People tend to be on first-name terms from the moment of introduction, and surnames are often hardly mentioned. Moreover, first names are relentlessly abbreviated, particularly in the media: Susan becomes Sue, Terrence Terry and Robert Bob not only to friends and relations, but to millions who know these people only as faces and/or voices.quoted in Burchfield, R. W. (1996). The New Fowler's Modern English Usage (3rd ed.). p. 512. ISBN 978-0199690367.