We've come up with the most comprehensive definition of an io game yet: an io game is a free to play, browser-based, casual game which has a multiplayer component, very few mechanisms, and minimalist graphics.
This definition may appear broad, but when used as a checklist when evaluating a game, the truth emerges.
The only one of these seven features that could be stretched a little further is the section on graphics. While 2-D minimalist graphics were a hallmark of early io games, we're seeing an increase in 3-D games, particularly those of the First Person Shooter variety. https://iogames-free.com