IPA: /dɪˈmɪn.jən/
KK: /dəˈmɪn.jən/
A territory or area that is controlled by a person or group; it can also refer to the power or control someone has over something.
The king ruled over his dominion with wisdom and strength.
Dominion is formed from "dominium" (meaning ownership or control) which comes from "dominus" (meaning lord or master) in Latin. The word refers to the territory or control that a lord or master has over a domain.
Think of a 'lord' or 'master' ('dominus') having 'ownership' ('dominium') over a territory — that's what dominion means.