Casual is a pretty broad term. Many people have their own definitions and it really depends on the person using the word. An example would be someone who rushed to the end-game content and has all the best gear for the current patch might use the term as a form of insult to anyone else who didn't do the same. A person who only has time to play the game maybe 8 hours a week total might use it to it's most literal definition.

I personally see the term casual as someone who has time to play, but has decided to not devote all their time in the game to the end-game content. I don't really see someone who plays casually being in a top FC on their server or in the world. A casual player, to me, is someone who could play the game for 8 hours a day and just enjoy the game or they could be someone who can only play an hour a day and still enjoy the game. It really depends on the person.