It's become the bogey man to toss out whenever a game does something you feel you don't like. Levels don't take months to gain? Casuals fault. Can solo some things? Casuals. Bad attitudes in game? Casuals (even though the worst offenders here are those who consider themselves hardcore).

It's a meaningless term now. It used to mean people who played short sessions a day, if that much, usually not too interested in progression or endgame and usually don't frequent forums. They are also by far the largest majority in any game, much to the dismay of people who consider themselves hardcore (who usually aren't really).