Core gamers buy 15 + newly released (less than 2 months old) titles (full games, not minis/arcade) a year. I'm sure most of us here buy more - but in marketing terms (y'know when you fill out one of those "how many games a month you buy surveys?) core is the top %35 - the 1+ game a month (and above) people.
That is what is meant by the "core" of the market. Sales. When talk goes of "pitching to the core" - it's these higher volume consumer gamers that effectively control the market. Irrelevant of genre. Although with so many FPS's released, there'd be a high crossover of FPS/Core gamers - though I doubt every single core gamer buys ONLY FPS. Hell if someone only buys Mario Kart6 & 7 , SMG1 & 2, New SMB, DKCR, OOT 3ds, We Dance 1, 2 and 3, LOZ TP, LOZ SS, Madden12, NFS, FIFA 12 and NHL 12 on Wii (in one year - all new) - they're "core".(I'm assuming none of these titles are budget/platinum/classic yet and actually come out on Wii - I could well be wrong - just trying to use an example of what some people call "casual" games). Often a "casual" gamer may buy one of these titles every 3 months or so - and that makes the buyer/gamer "casual" - not the games they buy.
In plainspeak - in the example of say - a coffee shop - the core market are the people who buy 3 + coffees a week. The weekly/fortnightly drop ins are "casual".
There's the debate of what makes a "core" game - and that's pretty irrelevant. A new game at retail is a new game. While we can talk of their "appeal to the core" - to actually call one game "core" and another "casual" (unless it's budget priced/small downloadable/Free To Play - eg. Zynga, Angry Birds, Uno)is purely subjective.
There's always open the debate of what a "hardcore" gamer is - and sure some guy who plays Mario 64 every day of the year and can speed run Metroid 1 in 45 minutes is undoubtedly "hardcore" - but if he's not contributing to the "new games" industry - he's not "core" from a marketing/demographic perspective, as he's not bolstering the core of the market.
OnTopic - How did we get off topic? ...um.. People - they buy stuff. I'm surprised at just how much they bought this year!
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