WordNet sense 1 (the state of being combined into one body):

coalition

WordNet sense 2 (a nuclear reaction in which nuclei combine to form more massive nuclei with the simultaneous release of energy):

nuclear fusion, nuclear fusion reaction

WordNet sense 3 (an occurrence that involves the production of a union):

merger, unification

WordNet sense 4 (the combining of images from the two eyes to form a single visual percept):

optical fusion

WordNet sense 5 (correction of an unstable part of the spine by joining two or more vertebrae; usually done surgically but sometimes done by traction or immobilization):

spinal fusion