Well after seeing this video ,probably this:
https://www.facebook.com/watch/?v=1324530338871497

Casual people game purely for short-term fun.

Hardcore gamers treat it as a job, hobby or project. - There are many people who have spent 10+ years playing nothing except World of WarCraft, they are clearly passionate about the game, I would conclude they are also "hardcore gamers" as they aren't just merely picking up a game, playing for a few minutes to get their dopamine fix and then moving onto something else.

So it's not about the types of games, how much you game or what platform you game on. It's about how passionate and involved you are with the medium.

This applies to other mediums like movies, television, music and even things like cars, motorcycles, boats and more.

Meaningless term used by insecure people.

I think one of the Nintendo Power editors said it pretty well many years ago.
I'm paraphrasing here: A hardcore gamer is someone who puts a lot of passion into the games they play.
I'd say that is pretty true. If someone plays Candy Crush or Madden or COD or Minecraft for a few hours or so a week without much passion other than the simple entertainment it's probably just a time waster or minor hobby.
And as far as gaming skill goes, pretty much any above average skilled player is probably a hardcore gamer. But not every hardcore gamer is skilled.

There are multiple types out there, but the ones I've come across over the years vary from:

• Ones dedicated to the grind, be it in an MP game to unlock gear/weapons or in SP games.
• Ones dedicated to the lore of any given game that they take interest in.
• Ones dedicated to becoming the best at something in a competitive game (Like Starcraft II or WoW PVP for example).

Generally it's people being driven by their love/passion for something so greatly, that they will dedicate months if not years of their lives towards said games/franchises. Normies/casual types tend to drop off said games after a time, but the hardcore types stick with entire franchises for years if not decades.

Most of us on this site are what one could call a hardcore gamer. We all tend to have our own core interests in our favourite franchises, be for the stories, the mechanics, the visuals, etc. Also hardcore gamer types tend to put in far more time into the games they love to play, be it on a daily or weekly, sometimes monthly basis.

That are different types of hardcore

The most easy to figure out is someone who put their time into being exceptional at a game or a group of games. They generally play competitively, we can say e-sports players are all hardcore

There are also the ones who use their time to breaking engines, discovering how they can change their interaction with games. Gaming for them is more than just playing, they try to understand the underlining mechanics of engines and game design. Think on guys who spend time datamining for example

I will say collector's and people who spend their time testing really obscure games can also be deemed as hardcore, specially if they follow developers interviews and make extensive record of games, styles, genres, etc, they are akin to cinema academics

Somebody who cares enough about video games to join a dedicated gaming community like this one. The people who call themselves gamers, in other words. People who participate in the culture of gaming. That by itself makes you more dedicated to video games than the other 90% of the population. All of us here are hardcore gamers or else we wouldn't be here.

(There are, of course, forms and degrees.)

I'd call those just core gamers.

Hardcore gamers additionally obsess about things like achievements, finishing games to 100% and knowing them in and out.

Also, speedrunners.

I divide people to four levels:
1. people who don't play games
2. people who play games just for fun (=casuals)
3. (basic/"real") gamers
4. hardcore gamers
(Of course, one person can spread to more than one class, I myself slip to casual in some stuff and reach hardcore in other, but am generally a type 3.)
I also define that 3&4 are "gamers" and 1&2 are not. Gamers are people who understand the phrase "I'm in the middle of a game" (=therefore I will stay here and finish it despite there being food/sex/fire).

Hardcore is harder to define as there are so many dimensions to it.
It's about time, effort and money spend on games.
It's about skill and difficulty.
It can also be about number and variety of games you play.

The only general definition of hardcore gamer I can think of is that they do things that over 95% of gamers can't or won't, either having such skill/knowledge of specific game or spending so much time and effort (possibly including one's limited money) on games.