A more thorough explanation:

Definition: An empire is a large group of territories or countries that are controlled by a single ruler, usually an emperor or empress.

Examples:

• The Roman Empire, which at its height included much of Europe, North Africa, and the Middle East.
• The British Empire, which at its height included colonies and territories all over the world, including India, Australia, and Canada.
• The Mongol Empire, which at its height was the largest contiguous empire in history, stretching from Eastern Europe to Asia.

These examples illustrate the definition of an empire because they all involve a single ruler or ruling family controlling a large group of territories or countries. In each case, the ruler had significant power and influence over the people and resources within their empire.