This term may be used both to refer to a math which opens its doors every hundred years, or to a member of such a math.
While it is possible for an Avout who joins a Centenarian math near Apert to live long enough to see the gates open a second time, most Avout will see one Apert and some may not live long enough, or at the right time, to see even one.
"Hundreder" is a slang term for an Avout belonging to a Centenarian math.
This is a term used to describe a breakdown in sanity which has been known to occur in Centenarian maths. It may also be used to refer to an individual whose sanity is in question.