A dynasty is a "line of hereditary rulers of a country" and an empire is an "extensive group of states or countries under a single supreme authority" (first results on google) Now I assume you have already looked these words up and are seeking an explanation however. A "dynasty" is distinctly different from an "empire." A dynasty describes the transferal of rule along a hereditary (familial) line (e.g. father to son, uncle to nephew). Examples include an extensive list of dynasties available here. An empire is generally labeled as such as the result of that state's imperialistic acts. These include the expansion of a state's territories, the cultural domination of other societies, exploitation of foreign lands and various other "imperialistic" acts (imperialistic being a loose term). Empires are also prevalent in history and include most notably the Roman and British empires that partook in the expansion of their states through various imperialistic activities. The distinction between "dynasties" and "empires" are that dynasties are a system of transferring political rule through based on heredity and an empire is generally a state that involves itself in imperialistic activities. It is easy to think about it this way: not all states that are ruled through a dynasty involve themselves in imperialistic activities and vice-versa (that is that not all states that are involved in imperialistic activities are ruled through a dynastic system of rule). To address your example it is because of the way power was taken. For example, when the Mongols took control of China from the Song dynasty in the 13th century they installed the Yuan dynasty. The reason for the difference of terminology is because sometimes a separate state would take control of another and install their own governance or dynasty however in other cases new dynasties (ruling families) would be installed by the removal of the preceding one (in some cases by their forcible removal) by different a faction within the state. The political status of these factions and their ascendance to rule is probably the discerning factor in their distinctions.