Definition is the most important information category. It is about defining every notion of period in a specialized way. “Cultural period” is a multidimensional concept about complex interrelations between cultural phenomena. The CIDOC CRM defines the basic notion of a period as: “"This class comprises sets of coherent phenomena or cultural manifestations bounded in time and space. It is the social or physical coherence of these phenomena that identify an E4 Period and not the associated spatio-temporal bounds. These bounds are a mere approximation of the actual process of growth, spread and retreat. Consequently, different periods can overlap and coexist in time and space, such as when a nomadic culture exists in the same area as a sedentary culture…". (Crofts, Doerr, G ill, Stead and Stiff 2004). This definition is based on a notion of coherence, a “unity criterion” in the sense of (Guarino and Welty 2002), that relates phenomena such as people following a certain life-style, political decisions, an economy etc. Th e CRM definition goes actually beyond cultural periods in the narr ower sense, but we shall adopt this definition here, intuitively restricting it to “cultural” periods.