At what point is it abuse or addiction? If one functions fine, what's the issue?
I would not know, I am not a doctor, but I would say the moment drinking impairs you and the dosage is above the recommended intake. When you develop an habit of drinking rather than having a drink with your meal because you want to accompany your food with some wine.
Someone who has developed an addiction does not "function fine" at all. They'll have secondary effects related to alcoholic habits and sometimes will resort to abuse the drink when situations overwhelm them, which usually worsens the case.
Take the smoking example. You would not consider someone who smokes a single cigarette every two weeks cause they felt like it at a party to be addicted (a rare case, but a case nonetheless), however someone that smokes a cigarette to "calm their nerves" every now and then throughout the day or even the week (a grey area to be covered individually case by case by a health professional) is something you'd start considering an addiction.
The lack of substance produces an effect on you: withdrawal. We see withdrawal clearly in very extreme cases of drug and alcohol, but it can also be faint and hard to notice. This "anxiety" smokers have to soothe with cigarettes is a form of withdrawal. Resorting to alcohol to "clear" your mind and forget your problems is a form of withdrawal.
So as it was said, "day drinking" is a name that wrongly describes the issue, because drinking during the day is no issue, but it is more common to see people drinking heavily at night (parties, events, disco, etc) rather that during the day, when alcoholics will continue drinking outside of events or whatever. The name comes after the "shock" of not expecting someone taking shots of vodka during the day rather than seeing them taking 10 shots of vodka while out with the lads.