Some one who plays games, but don't care about its history, its politics, its news. They may also don't care about deeper game mechanics.
Pokemon can be pretty casual and hardcore to the extreme for example: Pick it up and play it once for fun is a casual Pokemon master. The hardcore Pokemon master dudes breeds them over hundreds if not thousand of generations to the max/min with the eugenics of stats and techniques in mind. I didn't even know such mechanics even exists. I am therefore a Pokemon casual. However, I am a FPS hardcore utilizing and exploiting TTK, Latency, average reaction time, server tick, CoF Standing/Crouch/Strafing to the max. Casual FPS players accuse me of hacking all the time cause I kill them before I even render on their screen, etc.