In my mind, the phrase "hardcore gamer" mostly makes sense as a way to distinguish between gamer demographics:
- Casual Gamer - One who does play games, but primarily the kind of games meant to be especially approachable, especially games with no upfront price attached.
- Middlecore Gamer - One who plays games on a regular basis, but does not make it their primary hobby and/or limits themselves to a relatively narrow selection of "mainstream" titles.
- Hardcore Gamer - Can be defined in a few ways, but generally speaking is someone who makes games a primary hobby and/or spends a fair amount of time and money on a number of titles.
If these groups seem vague, that's because they should be. They're honestly more useful as a way to describe target audiences than individuals. For example, EA might develop their next sports game with the Middlecore audience in mind, aiming for sports fans who happen to play games enough to invest time and money into a full budget title.
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Alternatively, one can use the Casual to Hardcore spectrum to describe an individual's affinity/skill for a category of games. For example, I consider myself to be a casual enjoyer of puzzle games, since I don't invest a lot of time or money into those sorts of games, but a relatively hardcore RPG/Strategy fan, since I spend more time/money/skill into those genres. That said, this approach is even more difficult to nail down.