I think Carol's definition shows compassion towards those who have a one time mistake or lapse of reasoning. I think "relapse" implies a change in direction where "slip" implies a bump in the road.
I believe that slips and relapses are normal to recovery. It is only by the grace of God that it hasn't happened to me... Let's face it: It's normal for Alcoholics to screw up.
Back to the definition thing. I think the difference lies in the motivation and thoughts of the individual.
Relapse
Thinking about drinking and drinking.
Thinking "Nobody cares about me, I might as well start drinking again"...then drinking. A consious decision to go backwards in recovery, with no intention of setting it right again.
Slip
Not thinking about drinking, but picking up. It can happen. The individual feels bad about it and has the intention is to come clean at the next AA meeting, and continue in recovery.
These are only my thoughts on the matter, and everyone has an opinion.
chip