I think if a person keeps gambling away his money and spends a lot of time of gambling, then he is an addict in my eyes. Gambling should be done just for sake of entertaining yourself without wasting too much time or money. It shouldn't get into way of making decision for you and your family. You should be able to stop it at any time.
I think that most gambling addicts are pretty much invisible though, except for in real brick and mortar casino's.
Now with so many online casino's, it's pretty hard to notice gambling addiction. Except perhaps if you have a friend who's constantly gambling on his phone or something like that.
And you don't have to be gambling every day to be considered an addict.
If you gamble just once a week or once a month, but almost empty your bank account every time, you're also an addict