A dynasty is a sequence of rulers from the same family, usually in the context of a monarchical system, but sometimes also appearing in republics. A "house" is an imperial, royal or noble family, not always ruling. Historians periodize the histories of many states and civilizations, such as the Roman Empire, Imperial Iran, Ancient Egypt, and Ancient and Imperial China, using a framework of successive dynasties. As such, the term "dynasty" may be used to delimit the era during which a family reigned.