Cognomen
Black's Law Dictionary · Henry Campbell Black, M.A. · 1910
Black's Law Dictionary
In Roman law. A man's family name. The first name (prænomen) was the proper name of the individual; the secend (nomen) indicated the gens or tribe to which he belonged; while the third (cognomen) denoted his family or house.
In English law
A surname. A name added to the nomen proper, or name of the individual; a name descriptive of the family.