As far as I know, it is IE 12 (Trident 8.0), but they consider it a fork, which means technically you can have different features and support in Edge 1.0 and a future hypothetical actual IE 12. Mostly what they did was *remove* all kinds of backwards compatibility stuff from Edge, so that you can't trigger IE10/9/8/7/5 (yes, 6 was not a choice) rendering modes anymore. You can't use VBScript, ActiveX, and all kinds of other non-standard stuff. IE10+ is already a pretty decent modern browser, very much on par with Firefox and Chrome, so you should expect Edge to be just as good, as long as you're writing standard HTML5 code.
I tend to include in the headers of sites I write recently, because it removes the button in IE that lets users toggle backwards compatibility mode, but lazy people could previously use that tag with IE=8 or IE=5 to force IE into backwards compatibility rendering modes instead of updating their sites with standards compliant code. I guess that will no longer work with Edge. It will only render in HTML5 standards mode and nothing else now.