Emergence
When the whole is more than the sum of its parts. Wetness emerges from H2O molecules; consciousness emerges from neurons; markets emerge from traders.
Emergence is the phenomenon where collective behavior exhibits properties that no individual component possesses. No single water molecule is wet, yet wetness emerges from their collective interaction. No single neuron is conscious, yet awareness arises from their network. There are two types: weak emergence (in principle deducible from the parts, just computationally hard — like weather from air molecules) and strong emergence (genuinely novel properties that cannot even in principle be reduced — consciousness may be an example). The concept challenges reductionism: even with perfect knowledge of components, you cannot always predict systemic behavior. This is why complex systems continually surprise us — financial crises, ecosystem collapses, viral phenomena. Emergence is also deeply related to information: emergent properties often involve new informational structures (patterns, memories, meanings) that exist only at the macro level.