When it's said that the age of Earth is 4.6 bil years, what does that mean?

Question

Since it formed out of countless collisions with tiny rocks in a more-or-less continuous fashion, there doesn't really seem to be a clear distinction between "not Earth yet" and "Earth". So what is being defined with that "date"? The rock having Earth's present mass? A regular orbit around the sun? A certain core temperature?
Thanks

Answer 1

It means in effect, "This is the time in Earth's formation process by which enough of the stuff that makes up the planet had arrived, so it becomes meaningful to refer to it by name." Prior to that, it might have been called 'Proto-Earth'. The accretion process itself would not have been nearly as continuous and gradual as you might think. Once planet formation began in the protoplanetary disc, it went quite rapidly. In cosmological terms.

So, from your possible definitions, the choices would be "the rock having (mostly) Earth's present mass," and "a regular orbit around the sun"

Answer 2

Its just a guess someone made from the half life dating process. They say the oldest stuff found is that old so thats how old the Earth is. It really is nothing more than a guess that has evolved into a common belief.

Answer 3

Your inquiry makes no more sense than asking how old are you, since some scars and hair have different ages.