Considering I just used this one, I thought it the perfect time to define it.

cognomen (cog·no·men): / kog NOH muhn /
noun

(1) surname
(2) any name, especially a distinguishing nickname or epithet
(3) the surname or family name (third and usually last name) of a citizen of ancient Rome

[Irregular formation from Latin prefix co- (“with, together, auxiliary, subsidiary”) + nomen (“name”) meaning “added name.”]

Usage: [via] Although no one would ever mistake one of these geese for a swan (unless seen from quite a distance), I realized while watching them approach the shore that they indeed deserved that very cognomen.