What is an empire, how is it different than a city-state or a kingdom?

A empire is an extensive group of states or countries under a single supreme authority, formerly especially an emperor or empress. A kingdom is a country, state, or territory ruled by a king or queen, and a city-state is a state that with its surrounding territory forms an independent state.

An empire is when a country or state controls many different countries through conquest or allies. an example is the Byzatines or the Roman