A nation is basically a geographical community of human beings grouped together on the basis of common characteristics like language, culture, history, ethnic, culture and so on. A nation also is the shared identity of an ethnic group recognized as defined by the attributes identified by the different groups of individuals. There are many definitions of nationhood and while many are agreed upon some are rejected completely. A nation is generally considered to be a political entity having a constitution and government established by the citizens of the nation.
Nation-states are political entities having their own constitution and government established by the citizens of the states. These nations often have much more political power than the national identities of peoples within a nation-state as each state has a vote in terms of representation in the national legislature and so forth. Nation-states can have anywhere from one to twenty states in their boundaries with much less than that depending on the number of states in the boundary. In terms of the territorial integrity, a nation-state would have one state in the middle and any additional states stretching to the edges would be considered part of that nation-state.
Nationalism is a collection of various nationalistic views about membership of the world, political unity and territorial integrity among all nations of earth as represented by states or nations. Nationalism is a positive trait as it encourages cooperation among all nations, it encourages respect for other cultures, and promotes international solidarity through the means of cultural norms and language. As it stands it seems that nationalism in no way contradicts political pluralism and it is actually necessary to have both in order to promote a global civilization. Accordingly I would define politics as the interaction of forces within a nation-state that makes any polity viable providing that such polity can be brought into being through the consent of the governed through representative assemblies of the people.