It's where the marginal cost and the demand curve intersects.

边际成本与需求曲线的交点处

耶鲁公开课 - 博弈论课程节选

Let me finish the picture here; here's my demand curve, here's my constant marginal cost at c, and I want to use this picture on the left now to figure out what the monopoly quantity is.

先把图画完,这条是需求曲线,这个是边际成本常量c,下面我们通过左边的图像,来找出垄断产量

耶鲁公开课 - 博弈论课程节选