The key is that the person who is developing as an alcoholic starts by one drink because he 'needs one', and progresses gradually over time until the drink is essential.Since his tolerance of alcohol, and the speed with which he gets it out of his system, both increase, he requires more and more to reach a level as a 'cruising speed' the level he lives at just to function normally.He will pass through life,with blood-alcohol always at that steady level without others knowing,or suspecting, that he drinks.Finally he reaches the stage where drink completely rules his existence, he is living to drink. By this stage he'll lose his job,his wife or partner and his family and not appear to care, because 'they weren't worth it anyway' or 'that happens'.Then the world calls him an alcoholic, but the signs were there, and he was one, long before.An experienced person, a doctor maybe, can see the earlier signs far before that stage (hence their concern) Throughout, the person himself is in complete denial even in the face of overwhelming evidence (as it will eventually be).
Alcoholics can certainly go without alcohol ( I did for six months, George Best allegedly a year) or 'binge drink' and abstain but they are still alcoholics underneath.Some of us are lucky. George Best,Tony Hancock and many others were not. I was; so were Anthony Hopkins and others,but unfortunately we are likely to be the minority.
With luck, the person 'hits the brick wall' and can be helped successfully.Until then, if he accepts he has a problem that's a start but nothing is certain until he finally realises that he cannot go on as he is.