Definition Varys but this is how I define it

Casual Player: A player playing mainly for fun. They put most RL activities above the game. They have a limited playtime usually below 20hrs a week. (So combination of Time they are playing and mentality of the player)

Hardcore Player: A player who is serious about maximizing themselves and the group. They put many RL activites on the backburner and put a ton of time into the game to be the best(Example in XI waking up at 3am to camp fafnir, camping Tiamat for 24-72hrs to get claim, not going out with friends to make a scheduled Raid). They play alot sometimes upwards of 6-10hrs per day.

I do not think there is a direct definition. You can play 60hrs a week but not be serious about the game... I would not consider you hardcore in that case. You can wake up at all hours of the night to do something, skip RL things but have a semi limited play time... So hardcore mentality but more of a casual playtime.