A Catalyst is a person who functions as an agent of change for a White Prophet.
The function of the Catalyst is to redirect events such that the world is jostled out of its predetermined cycles and on to a better path. Essentially, a Catalyst prevents history from repeating itself.
Little is known about why certain people are designated as Catalysts. A seemingly unremarkable person may be recognized by a White Prophet and have their actions manipulated so as to effect a change the White Prophet has foreseen and desires.
A Catalyst is only identifiable to a White Prophet, but it is not known if every White Prophet identifies a Catalyst, nor if every potential Catalyst is guided as such by a White Prophet. It is also unclear if a White Prophet may have more than one Catalyst.