Quoted from an email originally sent to: Ten-Eight
The author writes:
Let's analyse Decimus Marcius Censorinus's name, shall we?
Decimus is the praenomen— what we English speakers would call the "first name" or "given name". It means "tenth", indicating that Decimus is the tenth son.
Marcius is the nomen— what we English speakers would call the "last name" (even though it comes second), "family name"”, or "surname". No, it is not a middle name in the modern sense. It instead identifies Decimus as being from the gens known as "Marcius", whose name was derived from the praenomen Marcus, which, in turn, was derived from the Roman God Mars.
Censorinus is the cognomen—a concept that doesn't really exist in Western society any more (that I know of). It identifies Decimus as being a member of a certain branch of the gens Marcii, and is what most people would call him.