An empire is a collection of smaller kingdoms or territories over which a powerful ruler or emperor exerts supreme power, often after waging war against smaller kingdoms.

The key features of an empire include:

1. Maintains an army to keep tributary states under control, expand the empire, or protect it from outside aggression.

2. Designs and maintains an administration with officials to manage territories, collect taxes, and maintain law and order.

3. Makes laws, issues currencies, weights and measures, and regulates trade.

4. Controls and regulates access to resources like mines, forests, agricultural produce, and manpower.

5. Encourages art, literature, religions, schools of thought, and centres of learning.

6. Maintains communication networks including roads, river and sea navigation, and other infrastructure.

The main difference between an empire and a kingdom is that an empire is much larger, comprising multiple kingdoms or territories that retain their own rulers but pay tribute to the emperor, while a kingdom is a single territory ruled by a king.

An empire has greater diversity of people, languages, and cultures, requires more complex administration, and exerts control over a wider geographical area than a kingdom.