There are no clear definitions and they can refer to either mindset or schedule. You can't really bring in definitions across games because each game and their community is different so the definitions are also different. It's best if you just find a group that fits your own circumstances and goals. You can have a groups with casual schedule and hardcore mindset, or the other way around. Generally the loose definitions tend to be as follows:

Mindset:
-Casual: We just play for fun. It's nice if we clear.
-Midcore: We want to clear before the next raid tier comes out.
-Hardcore: We want to clear as soon as possible.

Schedule:
-Casual: 2 days a week or less. Less than 3 hours a day.
-Midcore: 3 days a week. 3 hours a day.
-Hardcore: 4-7 days a week. 3-24 hours a day.

One particular aspect about schedules that muddies the definitions is that you can have a group that raids everyday until they clear a fight, but after that only farms the weekly reclears for like 2 hours once a week.

Generally you just want to figure out 3 things and ask these questions from the groups you apply to as well:
-What is your mindset?
-What is kind of schedule fits you?
-What do you want to do after your group clears?

Rather than trying put yourself into any of the categories, figure out what you want and find a group that suits your goals.

If you've found The Balance Discord, I'd take a look at the server listings there for your data center's raiding discord.