If "kingdom" refers to a sovereign state:
* Republic: A state where supreme power is held by the people and their elected representatives.
* Democracy: A system of government where supreme power is vested in the people and exercised by them directly or indirectly through a system of representation, typically involving periodic free and fair elections.
* Nation-state: A state with a single, dominant nation that controls the territory. This can be a kingdom, republic, or other form of government.
If "kingdom" refers to a realm or dominion:
* Province: A territory or region within a larger state.
* County: A division of a state or country.
* District: A region or area within a larger jurisdiction.
If "kingdom" refers to a biological classification:
* Phylum: A higher level of classification in biological taxonomy.
The best antonym for "kingdom" will depend on the specific context in which it is used.