@Graffito pointed a possible the answer when he mentioned the nobiliary particle:
A nobiliary particle is used in a surname or family name in many Western cultures to signal the nobility of a family.
However, the mentioned de, as well as the common Dutch van are not signs of nobility (contrary to the German Von). However, the wikipedia article continues:
However, in some languages the nobiliary particle is the same as a regular prepositional particle that was used in the creation of many surnames.
So it seems that at least grammatically we can refer to it as a prepositional particle, used in the same way as a nobiliary particle.
Of course, I doubt anyone would know what to fill in on a form if you use prepositional particle as a descriptor of a name field...
As far as usage is concerned, the prepositional particle is considered part of the surname. So Jan de Vries
has the surname de Vries
, not Vries
. In Belgium, the prepositional particle is even used when ordering names alphabetically.