If you describe a word as the operative word, you want to draw attention to it because you think it is important or exactly true
in a particular situation.
As long as the operative word is 'greed', you can't count on people keeping the costs
A small dram of whisky may be prescribed by doctors for those over 60 to help them
sleep. It is vital though to emphasize that 'small' is the operative word.
COBUILD Advanced English Dictionary. Copyright © HarperCollins Publishers