It’s that the singular point is the point in the neighborhood of which the differential relation dy/dx changes its sign . For example: vertex, relative vertex of a curve before it descends, so you will say that the differential relation changes its sign. It changes its sign at this spot, but to what extent? To the extent that it becomes equal; in the neighborhood of this point, it becomes equal to zero or to infinity. It’s the theme of the minimum and of the maximum that you again find there.