Imperialism is empire building.
Expansion occurs when one state is more powerful than are the obstacles to expansion. The obstacles may be other states or peoples, or they may be geographic or physical or technological obstacles. The central core of the empire may be a nation-state, or in ancient times, a city state or a tribe. European civilization experienced a period of unprecedented rapid expansion around the globe during the last third of the nineteenth century. European global expansion had actually begun in the fifteenth century, but the process greatly accelerated in the nineteenth century.