Pareto efficiency
If we look at what the economy produces, we might say it is on its Pareto Frontier or Production Possibility Frontier, meaning that it is making (or "producing") all it can with the things it has (such as machines, labour, land and ideas -- all called resources or factors of production). If this is the case, making more of one thing would mean making less of another; the resources are already being used to full capacity, so making more of one thing will take resources away from making another.