Unilateralism is any doctrine or agenda that supports one-sided action. Such action may be in disregard for other parties, or as an expression of a commitment toward a direction which other parties may find agreeable. Unilateralism is a neologism which is already in common use; it was coined to be an antonym for multilateralism, which is the doctrine which asserts the benefits of participation from as many parties as possible.
The two terms together can refer to differences in foreign policy approached to international problems. When agreement by multiple parties is absolutely required—for example, in the context of international trade policies—bilateral agreements (involving two participants at a time) are usually preferred by proponents of unilateralism.
Unilateralism may be preferred in those instances when it's assumed to be the most efficient, i.e., in issues that can be solved without cooperation. However, a government may also have a principal preference for unilateralism or multilateralism, and, for instance, strive to avoid policies that cannot be realized unilaterally or alternatively to champion multilateral solutions to problems that could well have been solved unilaterally.
Typically, governments may argue that their ultimate or middle-term goals are served by a strengthening of multilateral schemes and institutions, as was many times the case during the period of the Concert of Europe.
Unilateralism has had a long history in the United States. In his famous and influential Farewell Address, George Washington warned that the United States should "steer clear of permanent alliances with any portion of the foreign world". Many years later, this approach was labeled (by its opponents) as "isolationism", but some historians of U.S. diplomacy have long argued that "isolationism" is a misnomer, and that U.S. foreign policy, beginning with Washington, has traditionally been driven by unilateralism. Recent works that have made this argument include Walter A. McDougall's Promised Land, Crusader State (1997), John Lewis Gaddis's Surprise, Security, and the American Experience (2004), and Bradley F. Podliska's Acting Alone (2010).