Cognomen

The Cognomen, (Latin) while not a family name, was regularly passed from father to son as a name that said something about the family in a similar way to how the agnomen said somthing about the individual. It could be defined as the third name in a four name solution to Roman citizens.

Example

An example might be of a family having Gaulish ancestory and still identified as such, so they might add galli or a derivative thereof onto the family name. Likewise they might add something to indicate having red hair or the likes. Similarly, the name of a god the family acknowledged could be added, an industry or anything the family wished to declare about themselves. Therefore, the cognomen was not the family name, but extra information attached thereto upon which an agnomen could yet be declared.

Etymology

Nomen, Latin, literally translated "Name" but in light of the praenomen, the nomen should be considered as a family name. See also agnomen.

Cognitus, Latin, literally translated "the act of getting to know or become aquainted with".

Thereby the cognomen should be considered as a name by which one helps identify the family: a meaningful addition to the family name.