I propose we officially rename the terms "casual game" and "hardcore game" to those. Why? Because they are more accurate.  Any jackass can call a game he/she doesn't liike a casual game, because the term isn't well defined.

Here, the line is clear. If you can start the game, play for aroud five to fifteen minutes, and it doesn't matter how far you are, it's a pick-up-and-play game (or pickup game for short). If you haven't really done anything in that time, save for beating one level, it's a long session game (or any better name you can think up for it).

Some genres are either kind almost by definition. Puzzle games and Golden age games are pickup games. You can go for long sessions to get high scores, but that makes you a hardcore gamer, not the game itself. RPGs, on the other hand, are almost always long sessions, since the best you can do on a coffee break is either get through one or two cut scenes or complete a fraction of some level grinding.

Then there are games that are both. The most usual form is long session single player, and pickup multiplayer. This applies especially with fighting games and FPS (or TPS to include Gears of War). Another form is a game where some parts of the main game require intensive sessions, and some parts are pick-up-and-play. GTA is a prime example of this, but not the only one.

So anyone else think these are better and more accurate designations to these games?

EDIT: Edited the fourth paragraph for emphasis, and clarity.

Makes some sense. You are true about the casual being used as a somewhat insult for a game that is not liked.

Makes sense. Though I think it should also be stressed that no (or hardly any) games or gamers play exclusively casual or hardcore, if no one has said that yet. People can play games differently.

I don't get it though, i mean you can pick up and play just about any game. In RPGs you can play for about 5 minutes and only finish one battle but you did "pick up and play." Or in FPS's you play an online match against a team in like 5-10 minutes.

Yes, but a gamer can play a specific game hardcore or casually, depending on how they have the most fun with the game.  For example, I may play Wii Bowling for 20 minutes, so I am playing it casually.  Yet, my grandparents and their friends can play Wii Bowling for a long time (last Thanksgiving they made it to 2 hours!), so they are playing it hardcore.  Heck, sometimes playing casual or hardcore can differ at different times because of the game.  In Animal Crossing:  Wild World, I may only play for about a half hour in the winter when all the "good fish" and "good bugs" are not around.  But in the summer when all the beetles and good fish are around, I play for about 1 - 2 hours on a Saturday night.  These examples prove that the line between hardcore and casual is very vague.

Any attempt to cut all games in half won't work because there are too many exceptions and variables from the diverse ways in which different people play different games. I don't even like the word "gamer." I think we're all just people who like video games, and there are several types of games.

I think there are games based on reflexes, games based on strategy, games based on community, games based on cinema, and every possible combination of those 4 major gaming philosophies.

Pick Up-And-Play

Most of this kinds of games are interest but I tend to only played them for a brief amount of time a day or two(max).An example is when I first got my Wii console.My whole family was interest in Wii sports which was the novelty at the time.I played Wii sports first rather than Super Mario Galaxy which I also purchased.Due to the learning curve of the game and the fact that tennis was slightly "cooler"than playing with Mario.Everyone played along in games such as golfing,tennis and bowling(a favorite of everyone.)We made an agreement to do Wii sports weekends.But soon after a week the hyped were off and I ended up playing Wii sports by myself in two more occasions and that was it.

Middle ground(Bridge games)

Games such as "Guitar Hero III"and "Mario Kart Wii" can both either be played either way.The first one a unexperience player or someone who wants to have a fun time can picked it  up and play at a low difficulty such as "Easy"which might take him about 20-30 minutes to get used to the guitar peripheral or the Wii controls adn then he will probably already master the mode.And can also play against other players on vs mode.This game can be used as a party game with all the family participating.For other kinds of gamers that want to play "Guitar Hero III" by themselves.Can choose harder modes ranging from medium to expert.In order to beat all songs on expert this might take players a long time with various long sessions.The same can be said for Mario Kart Wii,someone can play it as a party game on multiplayer mode and play alongside their friends.Or one can choose to get a 3 star ranking and try to get the best times which is more challenging and will take a while.

Long session games

Such as RPG´S or adventure games,platform too.Then to get me hook more.They are offend addictive and require you to get items and solve puzzle e.g"Zelda"series "Mario" series.You can also choose to complete the game 100% which might take 200+ hours depending on the game and the objectives you need to do.

I donno whenever you buy A game you'll play it.

 Ok, but thats not what the OP was saying. Your is example is how a person plays the game that makes them hardcore. The OP's was what type of game it is makes the game hardcore.

what about GTA games?