General Synonyms:
* National
* Central
* Governmental
* Unified
* Nationalized
More Specific Synonyms:
* For government: Central, national, union, commonwealth
* For a system: United, centralized, consolidated
* For a structure: Hierarchical, layered, tiered
Examples:
* "The federal government is responsible for national defense." (National government)
* "The United States has a federal system of government." (Centralized system)
* "The company has a federal structure, with regional offices reporting to headquarters." (Hierarchical structure)
The best synonym depends on the specific meaning you want to convey.