Who is referred to as an emperor?

An emperor is specifically defined as the leader of an empire, which typically comprises multiple territories or nations under a single sovereign authority. This role involves not just governance but often also a level of cultural and military leadership that distinguishes it from other titles.

While a ruler of a nation may have similar governing responsibilities, they usually preside over a singular country rather than an expansive collection of different states or regions, thus distinguishing their authority from that of an emperor.

The remaining options refer to other roles or occupations that do not align with the political or imperial authority that an emperor holds. For instance, a person who performs ceremonies typically has a ceremonial role rather than one of governance. An expert in ancient history, although knowledgeable about past empires and their leaders, holds an academic or scholarly position, not a political one. Therefore, the most accurate description of an emperor within the context provided is as the leader of an empire.