In general, expansionism consists of expansionist policies of governments and states. While some have linked the term to promoting economic growth (in contrast to no growth / sustainable policies), more commonly expansionism refers to the doctrine of a state expanding its territorial base (or economic influence) usually, though not necessarily, by means of military aggression. Compare empire-building, colonialism, and Lebensraum.
Irredentism, revanchism, "reunification" or pan-nationalism are sometimes used to justify and legitimize expansionism, but only when the explicit goal is to reconquer territories that have been lost, or to take over ancestral lands. A simple territorial dispute, such as a border dispute, is not usually referred to as expansionism.
Geographic enlargement of the United States has been classed as expansionism; but geographic enlargement of the European Union has not, perhaps because the EU is viewed as an organization rather than a nation, despite the centralization of some governmental functions in the organs of the EU.