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; for such was the example of Octavian prior to his adoption by Julius Caesar wherein he used the cognomen of Thurinus indicating the township of Turi in Apulia located in southern Italy.

Likewise the family might add something to indicate having red hair or the likes. Similarly, the name of a god the family acknowledged could be used. Again an industry might be added such as the common English cognomens now used as last names of Smith or Thatcher. But ultimately the cognomen could be anything the family wished that helped describe themselves.

Of such the best commonly known example would be that of Gaius Julius Caesar manifesting the praenomen, nomen, cognomen, but never an officially attached an agnomen. Whereas to the family / gens nomen became attached the variant spelling of caesa denoting a cut (of a sword, knife or spear) making it a fitting cognomen that continued its use accross the successors of the Emperorship while not implying any genetic heritage but maintaining a consistent convention, a heritage and lineage nonetheless.

As such the cognomen is best thought of as extra information that the family wanted to declare about themselves.

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.