Disunion is from dis- ('apart, reversal') + union, from Late Latin unio ('oneness, union'), from Latin unus ('one'). The ancestry passes through Latin dis-, Latin unio, and Latin unus. The prefix dis- often marks separation, reversal, or negation in Latin and English formations, helping to explain the word's surface structure. In present use as a noun, it means "the breaking up of a union; separation or discord", keeping the historical formation close to the word's ordinary semantic work. Related words include disunite, union, disunity, and unite; derived forms include disunionist.