Economic Buyer
The person who controls the budget for your project — often not the person you talk to most.
The economic buyer can say yes to spending and can end the engagement. They are usually further from the daily work than your champion and care about different things: risk, ROI, and whether this makes their own commitments look good.
Worked example: a deployment can be loved by every user and still be cancelled because the economic buyer never saw a number they could defend in a budget review — which is why the quarterly business review is aimed at them rather than at the users. Gotcha: if nobody on the engagement can name the economic buyer by week three, that is a risk item, not an oversight.