"God's grace is critical for salvation in the Arminian view, 
but it cannot by itself guarantee that sinners will cooperate with it through their power of choosing.
The notion of human cooperation with God indicates that salvation is synergistic, meaning 'multiple agents(God and man) working(ergon) with(syn-) one another.'  
This is contrast with Calvinism, which teaches that salvation is monergistic, 
meaning 'one(mono-) agent working(ergon).' 
In this case, salvation is solely the work of God's grace."

(Scott Christensen, "WHAT ABOUT FREE WILL?")