Source : Webster's Revised Unabridged Dictionary (1913)

Cognomen \Cog*no"men\, n. [L.: co- + (g)nomen name.]
   1. The last of the three names of a person among the ancient
      Romans, denoting his house or family.

   2. (Eng. Law) A surname.

Source : WordNet®

cognomen
     n 1: 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" [syn: {nickname}, {moniker}, {sobriquet},
           {soubriquet}]
     2: the name used to identify the members of a family (as
        distinguished from each member's given name) [syn: {surname},
         {family name}, {last name}]
     [also: {cognomina} (pl)]