No, not really. Many casual games are not niche but that doesn't necessarily mean casual=not niche.

Non-niche simply means it has a diverse audience, not necessarily a casual one.

Memory, not quite.

Niche = Specific Market
Casual = Nonspecific market that transcends a specific definition.

doesn´t casual mean that it appeals to everyone? Sales haven´t to do anything with that.

The definition of niche, like you said, is that it appeals to a certain audience.

Therefore my deduction that not-niche=casual.

Edit: in simpler, more understandable words? 

My definition of niche (what I have learned)..

Niche = A small portion/piece of the total market where the product has sold on...

If you have a meeting at your company where you state your product has sold to 10-20% of the available market the people are going to laugh with you if you call it is niche;..

If MGS4 is niche....  What are games like Okami, SMT then?

I guess it all depends on what you think of the word "casual" but if that definition works for you, go for it.

I'm not going to start a semantical debate over every word used in this thread.

As a rough guestimate I would say that (typically) 90% of games are not sold to the die-hard fanboy, and are bought because they're high profile games on a system that is already owned by the gamer and are in a genre that these gamers enjoy; I would say that the most loyal fanbase for any game would (at most) represent 25% of all sales of that game.

When a system has a smaller userbase that is not growing as quickly there are simply less gamers to pick up the game because it is high profile, and thus the game sells less units. For the most part this is a trend we have seen for most PS3 titles todate, and the inverse trend has been demonstrated for most Wii and Nintendo DS games todate.

Okay, if we are going to pick and choose the appropriate usage of words even when the dictionary contradicts us, from this point forward whenever anyone uses the word "niche", I am going to assume they mean this:

1. A vessel adapted for various domestic purposes, and anciently for sacrificial uses; especially, a vessel of antique or elegant pattern used for ornament; as, a porcelain vase; a gold vase; a Grecian vase. See Illust. of Portland vase, under Portland.

If you buy MGS4, you are planning to use it to kill your dog. I better get the Human Society on speed dial, I have a lot of calling to do.

 Even more niche, I guess.

No, because niche has nothing to do with size.

Why is this so difficult to understand? Niche simply means that something has a distinct market. Nothing more.

Argh.

Yes I get it easily now

niche = ONE type of person will like it .... e.g. - Shin Megami Tensei - JRPG Hardcore gamers

No-Niche = MORE than ONE type of person will like it....e.g. GTA....people who like dricing games, fling games, shooting games....and so on