As is the belief that casual has to do with what content you're doing, and not how you approach the content.
Someone who logs on once a week or so and does some world quests and rares in ZM? Casual.
Someone who logs on everyday to ensure they hit up every single world quest, daily, and rare to maximize rep? Not casual
Someone who logs on once a week or so to knock out some +15s in the vault? Casual
Someone who logs on every day to fully fill up their vault? Not casual
Someone who logs on once a week to raid mythic for 3 hours? Casual
Someone who logs on everyday to grind out their 3000 ratings to buy 278 conduits, max fills vaults to get more options or currency to buy sockets, raids 3x a week, pushes thru heroic to get more chances at tier? Not casual