When you've quit smoking, but give in to temptation to smoke again, it's not a slip.
But when you smoke, you've made a conscious choice. A decision. An action you controlled. That's not a slip.
That's your addiction convincing you to take the wrong road.
Get back on track. Start back on the road you really want. But don't call it a slip.
No one pushed you and no one iced your path. Your own feet walked down that fork in the road.
C'mon back now and start over.
Start piling up those milestones for yourself as a better foundation.
[Edit to add: In my mind, and it seems in many others' as well, a "slip" denies personal responsibility. "Relapse" implies the active choice to ruin one's quit. Take it or leave it.]
[Second EDIT: I've closed comments on this post for now.]