" relates compositions of the liquid phase to the solid phase in equilibrium with the liquid phase and that indicates temperatures above which only the liquid phase can exist."
in other words, depending on your outlook on life, it's the state where it is neither solid or liquid,, or it is both solid and liquid. above this temp range it becomes liquid and below it is a solid.
I learned this term at Turley Forge long ago. Ive discussed it many times with welding instructors as well. This is the first time it's ever been a discussion of definitions.Instead of argueing here, I suggest you fire off an email or text to Frank. I have no doubt he can clear this up far better than me.
Correct me if I'm wrong, but this is what I've come to believe is "solid state welding". However It is not my area of great interest.
"Solid state welding processes are those welding processes in which no external heat is applied"