If the current is always flowing in one direction it is DC. The difficulty comes when the current is varying but never changing direction - I like to describe that as 'varying DC' to distinguish it from the simple case of constant DC.
Where the variation is small relative to the steady part of the DC, for example the ripple present on the output of a power supply which has been rectified and smoothed, I think most people will be happy to describe it as a form of DC.
Where the variation is large (but the current never changes direction) I say it is varying DC but some people will prefer to describe it as AC superimposed on steady DC - that is perfectly valid, it is just another way of describing it, and it is certainly the best description if the variation is so large that it is greater than the steady DC, making the current change direction.
Context is important in determining the best description when you are not dealing with simple situations.
I hope that helps!
John Hewes
electronicsclub.info