In United States full professorship is a path to take right after PhD, while in many European countries the preconditions to start a full academic career are different. The highest academic degree in some European educational systems is called “habilitation”, which is awarded not as a degree, but rather as a professional qualification for University professors.
Cumulative habilitation is another term used to indicate the research process, during which the person turns from one scientific field to another, e.g. from applied math to pure math. It is more than possible to hold a PhD in chemistry, while get the habilitation in biochemistry.
Habilitation equals to full professorship: a term more typical for US academic system. In Russia, for instance, it is fully equivalent to “doctor nauk” award, that is achieved after PhD studies and demonstrates even higher academic achievements.