In my opinion it is about play time. I would categorize most players into 3 groups:

Casual player: Plays 2-3 hours 2-3 days a week.

Enthusiast (Casual Hardcore): Plays 3-8 hours 3-5 days a week.

Hardcore player: Plays 8+ hours a day 5-7 days a week.

Keep in mind there are very skilled casual players and very terrible hardcore players.