The quality or state of working together effectively; joint or combined effectiveness or power.
From co- + efficacy (from Latin efficacia, power to produce effects). A rare or archaic term combining the prefix for joint action with the noun for effectiveness.
This is a genuinely obscure word, but it captures something important: the idea that when things work together, they have a special kind of power that's different from their individual strengths—yet we rarely have vocabulary for this collaborative multiplication of effectiveness.
Complete word intelligence in one call. Free tier — 50 lookups/day.