I use a strict dual definition with addendum.
First: A casual game is a game you can pick up and play with basically no prior experience and do reasonably well and ultimately high playtime is due to replayability, IE the game from start to finish can be completed in a few hours or even less.
Second: Alternatively it is a _type_ of multiplayer game of similarity from the first definition, however, also encapsulates games with zero 'start to finish' gameplay, IE party games.
Addendum: There are gamers that trend towards casual games but also play hardcore games. These players have a certain play preference which fits with casual games, however, they also like to play games with heavier depth and much longer 'start to finish' play times. They also play casual games in a hardcore manner, IE their total playtime of a casual game can be in the hundreds of hours.