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As they are used today, the two words are often used in the sense of state. “Empires are large, whereas kingdoms have generally been associated with the idea of smaller territorial dimensions. The figure of an emperor is of greater importance than that of a king”, says historian Newton Barbosa de Castro, from the Federal University of Juiz de Fora.
The issue of territorial extension is relevant, but perhaps the most important difference between the two concepts is another: the king governs his own people, while the emperor is sovereign over other peoples, conquered not only by force, but also by economic power or diplomatic efficiency.
Within an empire, several different ethnic groups can coexist, unified by a single administration and subject to the same authority. For this reason, normally a kingdom is a more stable political-administrative entity, while an empire is characterized by a constant attempt at expansion.
A good example to differentiate the two concepts is the English kingdom. With the conquest of territories outside Great Britain from the 16th century, it would come to be called the British Empire. That is until it lost almost all of its last colonies in the 20th century and went back to being just a kingdom.