I think relapse is a pretty subjective term here on SR, so I'm not even gonna try for objectivity.
I eschew the semantics - I think you're either drinking/using or you're not. Quantity is immaterial.
Recovery is a state of mind to me.
No such thing as a little bit pregnant.
But I know that's not the same as some other people's definitions, and I can live with that - what I think doesn't matter past my nose
For me, personally, one drink would be a relapse...cos it would never just be one drink.
D