To dominate something or someone means to have control or power over it.
Imagine being the captain of a sports team - you'd be in charge and others would do what you say. That's dominating, but not necessarily being mean or bossy. You're just leading and making decisions that help your team win.
Ruling; governing; prevailing
Example: The dominant party controlled the government.
The late period of the Roman Empire, following the principate, during which the emperor's rule became more explicitly autocratic and remaining vestiges of the Roman Republic were removed from the formal workings of government; the reign of any particular emperor during this period.
To govern, rule or control by superior authority or power
