What is Surname?

Legal Definition
A surname or family name is a name added to a given name. In many cases, a surname is a family name and many dictionaries define "surname" as a synonym of "family name". In the Western Hemisphere, it is commonly synonymous with last name because it is usually placed at the end of a person's given name there.

In most Spanish-speaking and Portuguese-speaking countries, two or more last names (or surnames) may be used. In Hungary, Hong Kong, Cambodia, China, Japan, Korea, Madagascar, Taiwan, Vietnam, and parts of India, the family name is placed before a person's given name.

The concept of a "surname" is a relatively recent historical development, evolving from a medieval naming practice called a "byname". Based on an individual's occupation or area of residence, a byname would be used in situations where more than one person had the same name.

A family name is typically a part of a person's personal name which, according to law or custom, is passed or given to children from one or both of their parents' family names. The use of family names is common in most cultures around the world, with each culture having its own rules as to how these names are formed, passed and used. However, the style of having both a family name (surname) and a given name (forename) is far from universal. In many countries, it is common for ordinary people to have only one name or mononym, with some cultures not using family names. Also, in most Slavic countries and in Greece, for example, there are different family name forms for male and female members of the family. Issues of family name arise especially on the passing of a name to a new-born child, on the adoption of a common family name on marriage, on renouncing of a family name and on changing of a family name.

Surname laws vary around the world. Traditionally in many European countries for the past few hundred years, it was the custom or law that a woman would on marriage use the surname of her husband and that children of a man would have the father's surname. If a child's paternity was not known, or if the putative father denied paternity, the new-born child would have the surname of the mother. That is still the custom and law in many countries. The surname for children of married parents is usually inherited from the father. In recent years there has been a trend towards equality of treatment in relation to family names are names with women not being automatically required or expected, some places even forbidden, to take the husband's surname on marriage, and children not automatically being given the father's surname. In this article, family name and surname both mean the patrilineal (literally, father-line) surname, handed down from or inherited from the father's line or patriline, unless explicitly stated otherwise. Thus, the term "maternal surname" means the patrilineal surname which one's mother inherited from either or both of her parents. For a discussion of matrilineal ('mother-line') surnames, passing from mothers to daughters, see matrilineal surname.

In English-speaking cultures, family names are often used by children when referring to adults, but it's also used to refer to someone in authority, the elderly, or in a formal setting, and are often used with a title or honorific such as Mr., Mrs., Ms., Miss, Dr, and so on. Generally the given name, first name, forename, or personal name is the one used by friends, family, and other intimates to address an individual. It may also be used by someone who is in some way senior to the person being addressed. This practice also differs between cultures; see T–V distinction.
-- Wikipedia
Legal Definition
A name which is added to the christian name, and which, in modern times, have become family names.

2. They are called surnames, because originally they were written over the name in judicial writings and contracts. They were and are still used for the purpose of distinguishing persons of the same name. They were taken from something attached to the persons assuming them, as John Carpenter, Joseph Black, Samuel Little, &c. See Name.
-- Bouviers Law Dictionary
Legal Definition
The family name; the name over and above the Christian name. The part of a name which is not given in baptism; the last name; the name common to all members of a famliy.
-- Black's Law Dictionary