Pierre Teilhard de Chardin coined megasynthesis in French in Le phénomène humain, completed in 1940, but published posthumously in 1956. He introduced the word to denote a vision of all the divergent streams of evolution during its fourteen billion of years of development converging in a megasynthesis of everything, at evolution’s Omega Point.
In the Hegelian dialectical process, synthesis denoted a new and higher level of truth, resolving the conflict between thesis and antithesis.
So when Life transcends the conflict of all opposites in the universal truth of the Principle of Unity in the Theory of Everything, it is natural to call this synthesis of all knowledge in all cultures and disciplines at all times a megasynthesis.
1959, from French méga-synthèse, 1956, from Greek megas ‘great’, from PIE base *meg-, and 1611, synthesis ‘deductive reasoning from causes or principles to effects or particular instances’, from Latin synthesis ‘collection, set, suit of clothes, composition (of a medication)’, from Greek sunthesis ‘composition, combination, a putting together’, from syntithenai ‘put together, combine’, from sun- ‘together’, from PIE base *ksun- ‘with’, and tithenai ‘to put, place’, from PIE base *dhē- ‘to set, put’.
From 1733 synthesis denoted ‘a combination of parts or elements into a whole’.