What is your definition of a Casual Gamer?

[forest-spirit]

Imo it's all about how you play games. Pretty much every game can be casual or hardcore depending on how you play.
You can sit down with Super Mario Bros. and just play through it for fun, which would be a more casual way of playing it. You can also try to get a high score, beat the game without dying, do speedruns etc. which would be a more hardcore way of playing. Generally speaking, of course.
Most gamers are a bit of both and extremely few are 100% casual or hardcore.

I'm also wholeheartly against the principle that hardcore gamers would be 'more' gamers than casual gamers, just because they prefer a different type of games. That's just something insecure people use to sound more 'elite'.