The Collaborative International Dictionary of English v.0.48:

Praenomen \Pr[ae]*no"men\, n.; pl. Pr[ae]nomina. [L., fr. prae
   before + nomen name.] (Rom. Antiq.)
   The first name of a person, by which individuals of the same
   family were distinguished, answering to our Christian name,
   as Caius, Lucius, Marcus, etc.
   [1913 Webster]