republican

Concise Oxford English Dictionary © 2008 Oxford University Press:
republican/rɪˈpʌblɪkən/

▶adjective

• 1 belonging to or characteristic of a republic.

• 2 advocating republican government.

  ■ (Republican) advocating a united Ireland.

• 3 (Republican) (in the US) supporting the Republican Party.

▶noun

• 1 a person advocating republican government.

  ■ (Republican) an advocate of a united Ireland.

• 2 (Republican) (in the US) a member or supporter of the Republican Party.

– derivatives
republicanism noun.