What is a cognomen?

A cognomen was the third name of a citizen of ancient Rome, under Roman naming conventions. Initially, it was a nickname, but it lost that purpose when it became hereditary. Hereditary cognomina were used to augment the second name (the family name, or clan name) in order to identify a particular branch within a family or family within a clan. The term has also taken on other contemporary meanings.

The term "cognomen" (sometimes pluralized "cognomens") has come into use as an English noun used outside the context of Ancient Rome. According to the 2012 edition of the Random House Dictionary, cognomen can mean a "surname" or "any name, especially a nickname". The basic sense in English is "how one is well known".