The next generation system of Microsoft. First came the X-box, then the X-box 360 (event though the system isn't a circle) and so it is assumed by most people that next will be the X-box 720. It is a reasonable name for the next gen. of X-box.
Sometimes used as a joke when asked if going to buy a 360.
Are you getting the X-box 360?
Nah, I'll wait for the X-box 720.