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1928, from multilateral + -ism.
also multi-lateral, 1690s, in geometry, "having many sides," from multi- + Latin latus (genitive lateris) "side" (see oblate (n.)). Figurative use by 1748. Meaning "pertaining to three or more countries" is from 1802. Related: Multilaterally.
Trade or diplomatic negotiations among several nations. (See diplomacy and recognition; compare bilateralism and unilateralism.)