I know the obvious answer is that it lets you resist magic but what counts as Magic in the game? Does Poison/Disease count as magic? Does Fire/Frost/Lightning? Or does Magic resist just count against effects that aren't covered by the other specific elemental resistances?

...far as I can see, it includes anything that is 'cast' on you, be it fire/frost/lightning or effects like paralyze and even a dragon's fire breath. That's why the wiki says "Resist Magic is probably the best defensive enchant you can use."

To my knowledge, only Fire, Frost, & Lightning count as Magic.
A Breton with 25% Magic Resistance will take -25% damage from Fire, Frost, & Lightning.
A Nord with 25% Fire Resistance will take -25% damage from Fire and full damage from Frost & Lightning.

Count Magic as something that does not exist in our real life. So shooting Fire, Frost, & Lightning out of your hand will be Magic.
Poison and Disease exist in real life, so they do not count as Magic. Nuts are poisonous to some people and there are diseases like.. you know.. diseases..

from the UESPwiki:

italics are mine

also,
Offensive magic must be anything that is cast directly or indirectly (runes) to cause destruction damage on your HP. (just my thoughts)

My plan, personally, is to get this Sheild of Solitude, disenchant it and use it as above (essentially stacking it with the other resist magic enchant) and to carry an extra light set of apparel that is charmed for free magic use (magic regen).

There are basically thee layers of magic protection:
1) Spell absorb from Atronach Stone or Alteration perk - This will either absorb spells or not, can often result in many spells in a row being absorbed, despite having 50% say. (Absorbs summons too, mod fix available on PC)

2) Magic resistance - capped at 85% iirc: will reduce all fire, shock, ice damage.

3) Elemental resistance - not certain if this is capped as I've seen companions show immune, but applied after magic resistance for another potential reduction.

More here: http://www.gamesas.com/index.php?/topic/1317202-complete-character-design-freedom-damage-resist-caps-and-ridiculous-damage-thread-8/

Poision/Disease are seperate.

Thanks for the replies everyone. As to this:


I have a random necklace I bought at the clothing store in Solitude (I was high teens for level at the time) that had 100% resistance to disease and I think I saw one that had 100% resistance to poison on another character I've deleted but that was over a lot of hours of gameplay so it may be ridiculously low chance of showing up.

I don't think it works that way.
I think that, for example, 30% of Absorb will reduce the effect of offensive spell by 30% and recharge your MP by 30% of that spell's MP cost (at least it should work that way).
The way you describe is way it worked in previous games.

epikal.bsf covered most of it.

Elemental Resistances are also capped at 85%.

Capping Magic Resistance at 85% should take precedence over Magic Absorption and Elemental Resistance.

Magic Absorption is "all or nothing". If you've got 30% Absorption from the Atronach perk, that means you've got a 30% chance to completely avoid all damage from an incoming spell and regenerate some Magicka in the process. Magic Absorption also works on Dragon Shouts, poisons, and lots of other negative status effects that bypass Magic Resistance.

Do you have any source to confirm this?
The description of some abilities which have this effect sound more like my version of the spell than yours.
For example:
'Atronach - Absorb 30% of the magicka of any spells that hit you.'
This doesn't say that there is 30% chance that it will absorb the spell, but that it will absorb 30% of it.

It's been tested extensively in one of the incarnations of the thread that was linked. It does indeed provide a % chance for all-or-nothing absorb, despite the description.

Krepaj: We've tested magic absorption ingame to verify, and Whiskeyjack is correct. Absorption is all or nothing, and it is possible for one character type to have 100% for 60 secs: the Breton. According to posters in that thread mentioned earlier, if you have over 50% passive spell absorption and then use the dragonskin power, you will be completely invulnerable to all magical effects of any kind for 60 seconds. It is a really, really potent ability for a properly perked and equipped character.

-Loth

Edit: FWIW, some of the in-game descriptions in skyrim are misleading/wrong, and spell absorption is just one of them. For example, the Shield Wall perks in the Blocking skill tree each add 10% per rank to your blocking, despite what the description text reads. That was discovered through testing too. If you take the time to read through the helper guide in the front of each thread incarnation, you might learn a lot about how this game really works.

Few things in Skyrim works as described

Thanks for clearing that out!

I was really happy when I saw the description since I believed that Bethesda dropped that lame 'all-or-nothing' thing.
It was really irritating in previous games and making it work like description says would make it more reliable and overall better imo.
I hope that they change that!