Aetherius:
Definition: Some say Heaven, some say it's Magic. I'm quite not sure on this one.
Description: Please provide a better definition.
Creation:
Definition: The process by which Lorkhan used/worked with Magnus and 8 other et'Ada to create Mundus.
Description: See the many myths.
Sun:
Definition: The Sun is the biggest hole to Aetherius made by Magnus on his forced departure during Creation.
Description: The definition pretty much sums it up I think.
Mundus:
Definition: The physical Realm created by Lorkhan and the Aedra. Represented as hub + spokes of the wheel in meta-physical view. Nirn is a planet/Realm of Mundus.
Description: Mundus is the solar system I think. It is the combination of Nirn/Earth as the main planed, the Sun, and eight other Planets which represent the Aedra. The space around all the planets and "inside" Aetherius is Oblivion.
Nirn:
Definition: The planet of mortals represented by hub in the meta-physical view of the wheel.
Description: Nirn is Earth in our imaginary solar system
Oblivion:
Definition: Place outside of Nirn and not part of the other planets, mostly composed of darkness/void with some cast-away creatia floating around and in some cases used to make Daedric Realms.
Description/Reasoning: My take is that physically, it's as close to "space" as possible, with some "planets" (Daedric Realms) and minor fragments of "matter". In the meta-physical view, Oblivion would be the area around the hub of the wheel between the spokes.
et'Ada:
Definition: Original spirits.
Description: No idea, can't imagine exactly what they are/were before Creation took place.
Aedra:
Definition: The 8 et'Ada who were tricked/convinced by Lorkhan to become part of Creation. They are the spokes of the wheel and are Planets in Mundus.
Daedra:
Definition: Spirit or creature (et'Ada) which didn't take part in creation of Mundus directly (e.g.: Prince) or indirectly ("lesser" Daedra). Daedra are considered not-part of Creation but they were changed by it. Locked in as it were.
Description/Reasoning: This one is a tough nut to nail more concretely. We don't really know the exacts about these enigmatic entities.
Daedric Prince:
Definition: Daedra who's concept/essence is a Sphere (see definition) which defines their purpose and place in the meta-physical wheel. They are easily considered Gods with power to equal such in regards to Mundus views by mortals. There are 16 known registered Princes with additional 1 oddity (Sheo/Jyg). Princes are the consciousness of their respective Spheres (see definition), and while their physical manifestation (the avatar) can be destroyed, they cannot.
Description: Daedric Princes are the representation of a Sphere (e.g.: destruction, madness, nightmares etc.). They are the embodiment of their individual Spheres. Can't really describe it much better.
Sphere:
Definition: Spheres are the basic concepts like destruction or madness behind Princes. Spheres are the indestructible part of a Prince, the very essence of his/her existence, purpose and limitations.
Description: In my view, the Sphere is the basis of any Prince. For example, for Sheogorath his Sphere is Madness, while his Realm (see definition) is the Shivering Isles. Sheogorath is the Prince, as the embodiment of Madness, he's consciousness behind Madness, but Madness the indestructible concept/idea/essence is what makes him what he is. Spheres are indestructible because they are ideas/concepts/purposes.
Realm:
Definition: Cast-away creatia used by a Prince to create a physical representation of his/her Sphere. Realms are the bodies of their respective Spheres. Since they are physical (exactly same as Mundus, from same material) Realms can be destroyed, but it has minimal consequence in the long run (the Prince rebuilds his Realm from new creatia which flows through Oblivion).
Description: Not much to add, the definition is quite clear I think.
I will edit this thread as people correct me, and also perhaps add more definitions as they come. One thing to note here is that Daedra don't fall too well into the model. First of all they didn't take part in Creation but are locked in by it. Also they have physical Realms, made of same material as Nirn is, but the Realms are not Planets as far as I know which kind of.. doesn't click.
Note specifically the difference between a Sphere and Realm. I'm not sure if the wording is correct but I'm fairly sure that these are 2 distinct things.
Also note that I say think Mundus to be more than Nirn, whereas in e.g.: UESP Nirn is a synonym for Mundus. I disagree, but have no evidence (at this time, will research it) to prove otherwise.