Realpolitik (German for "politics of reality") is foreign politics based on practical concerns rather than theory or ethics.
Otto von Bismarck coined the term after following Prince Klemens von Metternich's lead in finding ways to balance the power of European empires. Balancing power meant keeping the peace, and careful realpolitik practioners tried to avoid arms races. However, during the early 20th century, realpolitik was abandoned for the doctrine of "Weltpolitik" and arms races and alliances increased, culminating in World War I.
Realpolitik in many cases has been for the advancement of the national interests of a country over ethical or principled concerns.
One of the most famous proponents was Niccolò Machiavelli, best known for his Il Principe (The Prince). Machiavelli held that the sole aim of a prince was to seek power, regardless of religious or ethical considerations. The ideas were further expanded and implemented by Cardinal Richelieu and his raison d'etat in the Thirty Years War. The ancient Greek historian Thucydides, who wrote the History of the Peloponnesian War, is also cited as an intellectual forebearer of realpolitik. Other significant practitioners of realpolitik include Prince Metternich, Otto von Bismarck, and Count Camillo Benso di Cavour.
Various political science schools of thought rely on an analysis of political actions as realpolitik, most notably the Realist and Marxian schools. In the "realist school" of Anglo-Saxon Political Science of the late 20th century this term is mostly used as a synonym for power politics. The policy of Realpolitik was formally introduced to the Nixon White House by Henry Kissinger. In this context, the policy meant dealing with other powerful nations in a practical manner rather than on the basis of political doctrine or ethics — for instance, Nixon's diplomacy with the People's Republic of China, despite the U.S.'s purported opposition to communism and the previous doctrine of containment.
In Germany, the term Realpolitik is more often used to distinguish modest (realistic) politics from overzealeous (unrealistic) politics. That Prussia didn't demand territory from defeated Austria-Hungary was coined with this term, as was the sometimes very slow or indirect steps towards German unification under Prussia. Avoiding values like justice, nationalism or religious goals is in this definition only a temporary means to avoid losing everything. Today, the "Realos" of a political party don't mind making compromises on issues to ensure at least some progress, while the "Fundies" (fundamentalists) avoid compromises, even if it means they can't join the decision making. (In Germany, these expressions especially refer to wings of the German Green Party)