Diplomatic policy in which nations attempt to make peace by making concessions to an aggressive nation. Appeasement is often linked with the policies of Neville Chamberlain during World War II.
The most famous case of appeasement is the Munich Pact, in which Britain and France, under the leadership of British Prime Minister Chamberlain, conceded Czechoslovakia to Nazi Germany. The hope was that it would stop the aggression of Hitler and the Nazis, but it did not, and was largely seen as giving Germany a free pass.
Due to those failures, the policy today has a very negative connotation. Defense hawks regularly accuse opponents of appeasing the enemy.
Der Spiegel outlines how the strategy failed in the 1930s.