I don't like this trend, that would be like me calling all SNES games "SUPER Games" because most of them were prepended with the word "SUPER", but it's not a descriptor of the genre, just the nomenclature.
Do you have a better name?
How about "Browser-based"?
There are a lot of browser based games that are not "io games". The genre is much more narrow than you think.
No, besides browser-based it needs to be massively multiplayer, casual, without rounds (games are endless and you can jump right into an ongoing one) and without registration for it to qualify as an io game.