The term casual game is used to refer to a category of electronic or computer games targeted at a mass audience — typically with very simple rules or play techniques, a very low degree of strategy, making them easy to learn and play as a pastime. They require no long-term time commitment or special skills to play, and there are comparatively low production and distribution costs for the producer. Casual games typically are played on a personal computer online in web browsers, although the Wii and Nintendo DS< are also often referred to as a platforms catering to casual gamers. Casual gaming demographics also vary greatly from those of traditional computer games, as the typical casual gamer is older and more predominantly female.
What exactly "is" a casual game? The problem is that there's no good definition, and that any definition you try to make will have very obvious examples that break the rules.
In general, the definitions we came up with the web & downloadable games white paper are reasonably good:
Downloadable game: A game, typically less than 15MB, where the primary method of distribution requires download to, installation on, and execution from the end-user’s hard-drive. These games are almost exclusively available by downloading from websites. In the majority of cases they will have a trial mode, with the option to purchase the full version for unlimited offline play. Examples of downloadable games can be found at almost every casual online gaming site or games channel on the major portals. This does not include demos of video game or PC game titles that are primarily sold through retailers. This does include titles that are primarily available for download, even if the game is also distributed on CD-ROM.
Web-Based game: This term describes games that are launched via a web page with no prior installation of software required. This does not include games that are downloaded to the user’s hard-drive and run outside of the web-browser but it does include games launched from a web page that might require/install a general or custom ActiveX control. Common examples of this are the Flash™, Shockwave™, Java™, etc. games found on many game websites, as well as custom C++ games delivered via an ActiveX control.
If you combine the two you generally would get a casual game.
2007 Computergamesportal.com