Where the joined effect exceeds the sum of the separate ones. Synergy must be demonstrated; it is asserted far more often.
Greek συνέργια (synergia, joint work, cooperation), from συνεργός (synergos, working together), from syn (together) + ergon (work); the joined effect the word names was never assumed, it had to be worked for.
Where the joined effect exceeds the sum of the separate ones. It must be demonstrated in the work itself, far more often than it is honestly earned.