So I think a kind of key part of the term “casual” is its opposition to the term “hardcore”, which is deeply related to the cultivated consumer identity of “gamers”. A response I’ve heard to statistics about gaming demographics (like this pamphlet) have said that things like mobile games and Facebook games shouldn’t count. But the term “game” is extremely broad, just like the term “book” or “music”, and so on. Flappy Bird is by all measures a game.
The term “casual” refers more to the user’s relationship with games than it does games itself. The idea of a “hardcore gamer” is erroneously attributed to the type of game being played, even though a “hardcore gamer” could be one who plays RTS games or competitive FPS games. The question, for me, is more essentially: is it something the person does casually, or is a hobby? (as in this lol) Do you do Sudoku when you’re bored, or do you have bookshelves of the Nicoli books? Do you go on runs from time to time, or do you go on runs every morning and get the runner’s high?
I’m of the opinion that the divide between these types of play-style and these types of people is steadily thinning, especially due to ease of access. For example, “art house” directors like David Lynch are becoming more a part of the common discourse, instead of someone only “film buffs” are into. Similarly, “hardcore” games like, say, competitive shooters like Fortnite are explosively popular. (For the record, I think this is a good thing, and further breaks the divide between (scare quotes) “high” and “low” culture.)
So, all that being said, what I think of as “casual” is more dependent on how it demands your time and attention, but also importantly mastery. But this is also kind of complicated because games like Farmville are designed to demand attention, despite being ostensibly “casual”. I think Tetris, Animal Crossing and Sokobon/puzzle games games are great models for this stuff; they demand little from you but pass the time in a pleasant and meaningful way. I think of these games a lot like things like kendama or yo-yos; games that are engaging for the fundamental act of playing them, but don’t require dedicated mastery or dedicated attention.