I appreciate your effort, but I don't believe there is any formal definition. It's redefined per team, it's redefined by the team too if needed.
So people want an estimate (uncertain) based on a number that's also nebulous. It kills me.
Use time ranges, you get a sense for risk AND have an estimated delivery date.
The adjustment people make with numbers can still be made on a per team basis using time ranges, I don't see why we have to use a proxy