As far as I have always known, there is sort of this sub-space known as "The Essence" or "Ether" that the essence of things is sort of floating around in and tapped into. All living, natural things draw on this energy force to grow and live. When something decays (naturally), that force that held it together is returned to that Essence realm/pool. When someone's own sentient essence is released in death, it goes into that ether and swims about in the essence, slowly dissolving into the base essence to be re-used again in the cycle. (However, of late, that concept has been altered with some things like the Hall of Heroes, and The Dreaming where souls, copies of souls, or shadows of souls, reside and can be accessed, or removed.)
Empaths can channel that source and manifest the energy in our plane to do effects, and it also manifests in other such ways. Channeling a "boom" is gathering of that essence and using it to throw that energy at something and its violent release basically returns the essence back into the cycle--not destroying the energy, just altering it, or moving it from one place to another.
It is the destruction of essence entirely that actually known as "Necromancy" (not just the raising of the dead and undead, etc.). Phantera only has so much Essence, and destroying it is like removing water from the earth... the more it happens, the less there is until living things can't grow, etc.
Now, there are several other planes, realities, worlds that have shown up on different levels of Phantera before. For instance:
• There's the Elemental planes that exist in congruency, but separate from our own realm and may/may not affect our own plane's elements.
• There's also the Dreaming, or other realms that work with completely different metaphysical natural laws where entities may appear spiritual here, but would be material there...
• There are other worlds, other realms that are separate from Phanterra, or exist within its penumbra, or adjacent to Phanterra if not still within it, but separate from the realm of Haven somehow.
• And then there are even other timelines that have mashed and melded with the one we recognize, but may not have been native/intended for the one we are in (two separate pasts may have existed in one time, for instance.)
This all comes from my years of RP and conversations. It MAY be different based on who you talk to, but these are what I know to be general truths as far as I'm aware.
Phanterra is a place of metaphysical properties, or at least as we would recognize them. To a native of Phanterra, mixing herbs together to make a potion that can turn you into stone is just "natural science".
Perhaps a better definition (in/out of character) of "metaphysics" is: "Any effect or action that draws upon Essence from beyond the natural cycle to create an effect or act that manifests physically, or reacts with Essence."
As for what a character wants to call it?... that's entirely IC. If a race looks at Essence and calls it "Soujuice", and another thinks of the Essence as "Blood of the World", then so be it. Let folks call it whatever they want ICly. But hopefully I've helped OOCly.