In political science, a republican (lowercase r) is a person who advocates the establishment of a republic as a form of government, in contrast to a monarchist.
See also: republicanism
In politics a Republican (uppercase R) is a member, delegate or supporter of a Republican political party; for example the United States Republican Party, Fianna Fáil the Republic Party in the Republic of Ireland, etc. The politics of these parties varies widely with the context, although there is a common thread of support for an "independent" state.
In the Spanish Civil War, Republican was the name given for the side which fought unsuccessfully in defence of the Second Spanish Republic, and which were defeated by the forces of General Francisco Franco.
A republican in the context of Northern Ireland is someone who supports a range of Northern Irish parties, most notably Sinn Féin, from the nationalist community who possess an armed wing and engaged in what are seen as terrorist activity. Republican can also mean a supporter of the military organisation, such as the Irish Republican Army or the Irish National Liberation Army. Many of these organisations claim descent from earlier Irish republican movements such as the Irish Republican Brotherhood or the Easter Rising mounted in 1916.