Cognomen in a sentence as a noun

I wanted to know where the idea that it's common to view the cognomen as a surname came from.

OK, well, I am taking cognomen as surname, as is common when referring to him today, by nomen + cognomen.

It is like a nickname in that it is not assigned at birth and in that you have the option of referring to someone by cognomen or not.

The cognomen is what you as a modern Westerner would think of as a name -- it is the only part of a Roman name that identifies the person.

Cognomen definitions

noun

a familiar name for a person (often a shortened version of a person's given name); "Joe's mother would not use his nickname and always called him Joseph"; "Henry's nickname was Slim"

See also: nickname moniker sobriquet soubriquet byname

noun

the name used to identify the members of a family (as distinguished from each member's given name)

See also: surname