In definite integral definition we set the condition that y(x) function would be contiuous  in all integration interval points.  What happens, on the contrary, if some function becomes infinite in one point, for example in the lower bound of integration interval ? In such a point, we know that:

Well, keep on the function be continuous in all other point of integration interval. As you see in the side diagram, y=a/x2 function, for example, becomes infinite in x=0 point. If x1 integration lower bound would coincide in such a point, i.e. would be x1=0, the integrale definition should not can be applied. Look now how we can overcome this obstacle giving a meaning to such a case too.