Superadditive consequence is a phrase used by our authors to describe something, in popular parlance, "that is more than the sum of its parts." It is a foundational philosophical concept in the cosmology of the 5th Epochal Revelation.
This is beautifully described in [12:9.3]: "Not only in the realms of life but even in the world of physical energy, the sum of two or more things is very often something more than, or something different from, the predictable additive consequences of such unions. The entire science of mathematics, the whole domain of philosophy, the highest physics or chemistry, could not predict or know that the union of two gaseous hydrogen atoms with one gaseous oxygen atom would result in a new and qualitatively superadditive substance—liquid water."
But this concept extends far beyond the physical realm. Mind is a superadditive consequence of the relation between matter and spirit, for example. The Conjoint Actor is a superadditive consequence of the association of the Universal Father and the Eternal Son. The ideal union of a husband and wife produces superadditive consequences in realms of problem solving and the art of living, and of course procreation of children.
And so on.
This is a fundamental cosmic pattern that repeats itself all over The Urantia Book, once you are attuned to noticing it.