phrase → operative
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 down. ⇒ 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 © Harper Collins Publishers