The acronym MAIN most commonly stands for Militarism, Alliances, Imperialism, and Nationalism, especially in the context of explanations for the causes of World War I. However, MAIN can have other meanings depending on the domain, such as Main Street Capital Corporation or various network/infrastructure acronyms, so the intended meaning may vary by context. If you can share the specific field or usage you have in mind (history, technology, finance, etc.), the answer can be pinned to that sense.