Relapse is not part of recovery, it is part of addiction. There are people who manage to grab onto recovery and never have a relapse. Granted, many people do, but again, that isn't recovery, that is addiction.
Just my opinion, but to me, a slip would be if someone had one drink, realized what they had done, catches themselves and goes back to not drinking. A relapse, again, IMO, is when someone continues drinking after that first drink, even if they plan to start over "tomorrow." Others may look at it differently, but in my opinion, anyone who has more than one drink while in recovery, does so purposely and I would consider that relapse. I try not to get too caught up in words like that because any time a person in recovery has a drink, the potential for long term relapse is great.