prenomen

Fernald has no entry of its own for prenomen; he lists it under name.

Where he writes about it:

Among the Romans the prenomen was the individual part of a man's name, the "nomen" designated the gens to which he belonged, the cognomen showed his family and was borne by all patricians, and the agnomen was added to refer to his achievements or character. — from name

In modern use, a personal name, as John or Mary, is given in infancy, and is often called the given name or Christian name, or simply the first name (rarely the prenomen); the cognomen or surname is the family name which belongs to one by right of birth or marriage. — from name