The fact that an empire is controlled by an individual (emperor/empress) does not mean a monarchy heads an empire. Before an empire emerges, it will pass through different stages. Now, this stage starts from it being a kingdom. So, when the kingdom begins to dominate other kingdoms, its influence will expand over conquered kingdoms and territory. At this stage, the king or queen of the conquering kingdom title will be elevated to emperor or empress because they now have other kings and queens under their control.  So, while an empire can start from the stage of the monarchy, when the kingdom increases in size to the point of annexing other territories, it becomes an empire. While the head of the empire becomes emperor/empress, a title higher than king/queen. For example, Queen Victoria of Britain is also addressed as Empress of India when the latter was under British control.