Isolationism is a policy by a country of avoiding political and/or economic entanglements with other countries.
It may be particularly associated with, or made easier for, countries already isolated/protected to some degree by geography, such as due to being surrounded by large bodies of water and/or mountains. One example was Japan, before the "Empire of Japan" time period.
Isolationism may refer to a broader concept than non-interventionism, such as also opposing (some forms of) free trade with or foreign "voluntary" cultural influences from other countries. However, opponents of what is only non-interventionism or peace activism may prefer to label their opponents as "isolationists", possibly viewing this as a more a negative term. In particular, the foreign policy before the United States entry into WWII may be labeled as "isolationist", implied to somehow have contributed to the Holocaust, justifying various later coercive interventions, in particular against perceived enemies of Israel.
Globalists may use the term as being synonymous with any kind of opposition to globalization.