Let's not get reactionary, let's try to actually untie his logic.

I just think that the definition of "core" games is silly. What matter is that a game is fun, right? At least, that's what SHOULD matter. What IS a core game?

lol I actually never even noticed that.  Perhaps because I have drank a few brews.  Defence = Defense right?  Just a US - England difference.  Just like color/colour, flavor/flavour (our - or), etc.  Have you ever heard a British person pronounce Aluminum?   Wait a sec... I don't think Defence is even a word in England...

I really want to hear a British person pronounc "Aluminum", now.  If it's half as funny in real life as it is in my head, it's going to be awesome!

Record music with Vocaroo >>

There are no "Core" games unless you count the main big named titles/franchises like Super Mario, Final Fantasy, GTA, Resident Evil, Halo, etc. Its a just term coined in the internet to piss off the weak hearted fanboys in all circles and forms.

Also imo, the definition (or it should be) of a hardcore gamer is a gamer who plays video games without having a one sided preference of a brand/corporation/developer/etc. Most people confused hardcore with violence/blood, although most of the big franchises have those themes, not all of them are like that.

I understand what you are saying, but I personally want games that go beyond just being fun. Games with great story, sound, art style, etc. When all of those things are combine in a single package then it becomes a "core" game imo. Games like Zelda, Metroid and the like give me those type of experiences. 

You can make a clearcut definition for "core games", if you divide the market into "core" and "expanded audience".

"Core" will refer to any type of game that has been present and successful in the previous generation. In other words, the "core" is the bread and butter of the industry, the games that are known to sell, the market that can be analyzed because it already exists. For example, it's established that FPS sell, so new FPS IPs are automatically "core" games as well.

"Expanded audience" refers to any type of game that opens up a new market and, well, expands the video game population. In the following generation, "expanded audience" games will be absorbed by the "core", because they are now also known to sell. For example, Wii Fit was a new thing to the market in the seventh generation, but in the eighth generation fitness games shall be considered "core" as they are now a known market and it will remain "core" as long as they keep being made and keep selling.

There's actually a third category and that would be "core" games that stop being made for a generation or more, but then return. A good example of this would be New Super Mario Bros. Wii which can be considered an "expanded audience" game, but is now moving back into the "core" category for good with NSMBU.

However, most people use a "hardcore" and "casual" context which simply describes games they like and games they don't like.