The word 'union' entered English in the 15th century from Old French 'union', which itself derived from the Latin 'unio', meaning 'oneness' or 'unity'. The Latin term stems from 'unus', the word for 'one', which has a deep Proto-Indo-European root óynos, also signifying 'one, single'. The core logic of the word is the concept of separate entities being brought together to function as a single, unified whole. This literal sense of making 'one' from many evolved into its modern abstract meanings, encompassing political alliances, marital bonds, and organized labor groups.

Structure: uni (one) + ion (noun suffix) 'uni' originates from Latin 'unus', meaning 'one'. It functions as a combining form indicating singularity or unity. 'ion' is a suffix of Latin origin, used to form nouns denoting an action, process, or state.