Nomen

The second part of the Tri-Nomina is the nomen. Nomen means known or name. This was the only name most women had at that time. A praenomen might have come later on in her life. It indicated the gens from which a person came from. A gen is a group tracing common descent in the male line.   Unlike us the Romans usually called each other by their  “middle” name since praenomen were very similar. It would normally end in an –ius and later on -ianus was added. Using the same example we had in the praenomen, let’s try to find the nomen. If you forgot we used the name Publius Vergilius Maro. We know that nomen means name and to find its location we call it the “middle” name so Vergilius is the nomen.

http://ancienthistory.about.com/od/nameetymologies/p/Learn-The-Parts-Of-The-Roman-Name.htm