From Latin cognōmen, from con- + nōmen.

• (America) IPA: /kɒɡˈnoʊ.mən/

cognomen (plural cognomens)

1. (historical) A personal epithet or clan name added to the given name and family name of Ancient Romans.

   Julius Caesar's actual name was Gaius Iulius Caesar. Gaius was his praenomen or forename, Iulius his nomen or surname, and Caesar his cognomen, denoting which part of the Iulius family he belonged to.

   • 1886 October – 1887 January, H[enry] Rider Haggard, She: A History of Adventure, London: Longmans, Green, and Co., published 1887, →OCLC ↗:

     "Five hundred years or more afterwards, the family migrated to Rome under circumstances of which no trace remains, and here, probably with the idea of preserving the idea of vengeance which we find set out in the name of Tisisthenes, they appear to have pretty regularly assumed the cognomen of Vindex, or Avenger."

2. (literary or jocular) Synonym of nickname, any epithet used similar to the Roman cognomina.
   • 1838 (date written), L[etitia] E[lizabeth] L[andon], chapter XVIII, in Lady Anne Granard; or, Keeping up Appearances. […], volume I, London: Henry Colburn, […], published 1842, →OCLC ↗, page 237 ↗:

     Her husband was evidently a sensible man, and he might have given his wife a little more sense than she could have derived from her downright father and her silly mother-in-law, who were really as great a pair of noodles as ever were exhibited in the pages of a modern novel, under the cognomen of "amiable rustics."

3. (literary or jocular, uncommon) Synonym of surname, a family name.

• (Roman clan name or epithet) surname (increasingly uncommon)

• German: Nachname
• Italian: cognome
• Russian: фами́лия

• German: Cognomen, Kognomen

• Italian: cognome, soprannome
• Spanish: cognomen