I thought an empire is created when you occupy other countries and impose your system upon them. Like Rome was an empire because it stretched way above the borders or rome, or Mongolia, or persia, though maybe I have a wrong definition of empire.

But it seems that wikepedia agrees with me:

"Politically, an empire is a geographically extensive group of states and peoples (ethnic groups) united and ruled either by a monarch (emperor, empress) or an oligarchy. Geopolitically, the term empire has denoted very different, territorially-extreme states"