There are several different things to consider here when trying to classify someone as being a problem gambler.
A compulsive gambler (including poker players) is someone who gambles without the ability to stop themselves from doing so, even when it harms themselves or their loved ones. Such as spending too much money, spending more because you lose in an attempt to win it back. Lying to loved ones about your playing habits. Etc.
A
gambling addict (including poker players) is someone who gambles because it gives them an emotional rush. They continue gambling, not because they win, but because they crave how it makes them feel. They don't care if they lose because they love the rush of playing.
An obsessive gambler (especially poker players) is someone who, well, obsesses about gambling. Everyone knows what obsessing about a game or a hobby is and an obsessive gambler, or poker player is someone who obsesses about this. The difference between an obsessive person and the two categories above is that obsessive people don't do it because they have to and don't do it even though they always lose money, they just do it because it's what they love to do. Now, that doesn't make it a good thing. If you obsess about something it can very easily damage yourself or your relationships with loved ones, but that is true of obsessing about any game or hobby.
Finally a casual gambler is someone who only gambles within reason and only does so a reasonable amount of time. Now, that doesn't mean 1 or 2 hours a day max. A professional poker player could play 40-60 hours a week and it is reasonable because it is their job. It becomes unreasonable when it starts to adversely affect you financially or socially. For instance, gambling more than you can afford, neglecting time with your significant other to gamble, or staying up late to gamble and not being able to work well the next day would probably all be symptoms of one of the above 3 categories and would be considered unhealthy.