A Kingdom is a country ruled by either a king or queen. It is usually ethnically homogenous with a common language, history, culture, and sense of identity. There should be consensus among the people that the monarchy has a de jure right to rule over them. (E.g. the legend of the sword in the stone to find the rightful king of England.)An Empire on the other hand, is a large multi-ethnic state ruled by a central authority, often through military force. It could consist of multiple kingdoms and other political entities, but they must all be subservient to the emperor.  It is also usually defined by overseas/foreign territories. For example, the British empire was formed from the kingdoms of England, Scotland and Ireland, plus the numerous British colonies. The British Empire is widely accepted to have ceased with the return of Hong Kong, it's last major colony, to China in 1997. Other examples of empires defined by their overseas possessions include the Spanish, French, and Portuguese empires. Many land-based empires didn't have overseas possessions per se, but ruled over disparate ethnicities and cultures. Examples include the Romans, Mongolians, Ottomans, and the Zulus.The Holy Roman Empire is an interesting case. Voltaire famously claimed it was "neither holy, nor roman, nor an empire". This was because the HRE was a) ethnically homogenous and b) had no central authority. The HRE emperor had little real power over member states, who functioned as sovereign territories that could even wage war against each other. After the HRE disbanded, the new German nation was rightly called the "German Empire", characterized by strong authoritarian rule from Prussia and  overseas possessions with non-Germanic populations. Another interesting example is China. Qin Shi Huang is known as the first emperor of China, even though the "idea" of China had existed for hundred of years prior with the Xia, Shang, and Zhou dynasties. Prior to the Qin dynasty, "China" was basically a collection of semi-autonomous kingdoms that were culturally similar but with their own armies, laws, languages, and customs. Qin assimilated the other kingdoms, dismantled the title of "king", centralized all authority, appointed his own provincial governors, standardized laws and language, and promoted himself to "Emperor". China, although ethnically homogenous and possessing a strong national identity, thus became an "empire", simply because it's ruler was too badass to remain a humble king.

Empire vs Kingdom

Introduction:
Empire and kingdom are two different forms of political organization that have existed throughout history. While both empire and kingdom refer to territorial entities ruled by a monarch, there are distinct differences between the two.

Definition:
- Empire: An empire is a large and powerful political entity that extends its rule over a diverse range of territories and peoples. It is usually characterized by the centralization of power and the dominance of one state or ruler over others.
- Kingdom: A kingdom, on the other hand, is a smaller political entity that is ruled by a monarch. It is typically characterized by a hereditary monarch who exercises authority over a specific territory and its subjects.

Size and Territory:
- Empire: Empires are generally larger in terms of territory compared to kingdoms. They often encompass multiple kingdoms or regions, which are brought under the control of a central authority. Examples of empires include the Roman Empire and the British Empire.
- Kingdom: Kingdoms are usually smaller in size and are limited to a specific territory. They may consist of one or more cities or regions, which are governed by a single monarch. Examples of kingdoms include the Kingdom of Saudi Arabia and the United Kingdom.

Political Structure:
- Empire: Empires tend to have a more complex political structure compared to kingdoms. They often have a hierarchical system of governance, with provinces or regions ruled by appointed officials or governors on behalf of the central authority. The ruler of an empire is usually referred to as an emperor or empress.
- Kingdom: Kingdoms have a simpler political structure, with power concentrated in the hands of the monarch. The king or queen is the supreme authority and makes decisions regarding governance, laws, and policies.

Relationship with Other States:
- Empire: Empires often expand through conquest and bring other states or territories under their control. They may exercise dominance over other kingdoms or empires, imposing their rule and extracting resources or tribute. Empires may also have vassal states or client kingdoms that are subordinate to their authority.
- Kingdom: Kingdoms may form alliances or engage in diplomatic relations with other kingdoms or empires. They can exist as independent entities or be part of a larger empire, depending on the historical context.

Conclusion:
In summary, the main differences between empire and kingdom lie in their size, territorial scope, political structure, and relationship with other states. Empires are larger and more complex political entities that exert control over multiple territories, while kingdoms are smaller and more focused on a specific region.

Emperors are much bigger than kingdoms