What is a mother's maiden name?

In the English-speaking world, until recently it was the universal custom that upon marriage the woman would take the surname of her husband. Before marriage, she would use the surname of her father.

When a woman was not married, she was a "maiden," a woman who has never had sexual relations. So the "maiden name" of a woman is the surname she used before marriage, usually that of her father. Your mother's maiden name is the surname your mother used before she married, and is thus her father's (your maternal grandfather's) surname.

Before computers were widely used for activities such as genealogy and fan gossip, knowledge of the maiden name of one's mother was not readily available to people outside the immediate family. That name was therefore used as a security device by banks and other institutions. When you opened an account, you gave them your mother's maiden name. Later, if you needed to change the account, your knowledge of your mothers maiden name helped to assure them that you were, in fact, the valid account holder. Now that genealogies are widely posted on the Internet this is a much less secure method of identification.