Oblivion:Spell Absorption

Spell Absorption is an M% chance to convert all of the energy from a magical effect into your Magicka points and avoid its potentially damaging effect. For example, if you had 50% Spell Absorption, half of all spells (on average) cast at you would simply refill your Magicka, depending on the strength of the spell, instead of otherwise affecting you.  It applies not only to spells cast at your character, but also to magical damage caused by enchanted weapons.  Beneficial spells cast at your character (such as blessings at altars) will also be absorbed.

Characters with the Atronach birthsign have permanent 50% Spell Absorption.

Notes
The point value is the percentage chance any single spell will be absorbed.
Even with a 100 pt Spell Absorption effect, magic effects reflected back at you will still hit.
Even if you have the Atronach birthsign, custom Spell Absorption enchantments are not possible: this effect is disabled at the Enchanting Altar. However, the enchantment is possible through the use of Sigil Stones.
100 pt Spell Absorption is easiest to achieve for Atronach characters, who start out with 50 pts.  Other pieces of equipment that can be combined to get the extra 50 pts include:
Spelldrinker Amulet is a leveled item that provides up to 26 pts.
Sorcerer's Ring, available as random loot, gives 25 pts (plus 25 pts Fortify Magicka).
Several Shivering Isles magical items and Vanilla magical items available as random loot provide 24 pts (Cursed Absorbing Gauntlets and Cursed Magebane Gauntlets) and 20 pts (Amulet of Absorption, Greaves of Purity, and Magebane Greaves) respectively. Wearing the Gauntlets drains 10 Magicka pts. The Amulet of Absorption is particularly useful because it is available starting at level 9.
Mankar Camoran's Robe is a leveled item that provides up to 20 pts (plus up to 10% Reflect Damage).
The Spellturn Cloak from the Unearthing Mehrunes Razor plug-in has 15pts Spell Absorption at level 20.
Custom sigil stone enchantments provide up to 15 pts.
Helmet of the Mage, available as random loot, gives 14 pts (plus 20 pts Fortify Magicka).
The Atronach Stone provides the Arcane Well Greater Power, with 30 points Spell Absorption for 120 seconds (and Fortify Intelligence 10 pts).
Llivam's Reversal is an Expert level spell only available through the Spell Tomes DLC which provides 10 pts Spell Absorption for 20 seconds, together with 15 pts Reflect Spell for 20 seconds.
Your own Telekinesis spells are affected by any active Spell Absorption effects.  Therefore, your chances of successfully casting a Telekinesis spell are reduced (or impossible at 100 pt Spell Absorption).  If unsuccessful, you absorb magicka equivalent to the base cost of the spell (which can be exploited to increase your magicka, see following tips).
Tips for using Spell Absorption to regenerate magicka include:
Summon an enemy who attacks using spells.  Attack the summon three times so it turns on you, then absorb its spells.
If your Mysticism skill is 33 or higher, casting Telekinesis spells will increase your Magicka reserves if the spell is absorbed.  At 100 Mysticism, your Magicka is increased by five times the cost for you to cast the spell.  For example, Remote Manipulation at Mysticism 100 costs 5 Magicka to cast, but you absorb 28 points of Magicka.
You cannot use this exploit to fully recharge your Magicka, however, presumably because you actually absorb the spell before expending your Magicka.  In other words, if you have 450 total Magicka, casting Remote Manipulation at 100 Mysticism will never increase your Magicka past 445 (450-5).
Beneficial spells as well as attacks can be absorbed, including blessings such as those provided by chapels and Wayshrines.
The blessing cast by The Lucky Old Lady statue in Bravil is particularly valuable, since it can be activated as often as you like (unless you have completed the Dark Brotherhood quest line).
Stand in front of spellcasters who are practicing spells, for example the mages in the Arcane University's Practice Hall.
Take advantage of places with constant effect damage spells, for example, the frost damage in Frostfire Glade. Remember that this only works with external factors and will not work with custom enchanted apparel.
If you clear the area of daedra first, the fire turrets in Planes of Oblivion can be useful for this purpose.  However, damage you take from lava in the planes cannot be absorbed, as it is not considered a magical effect.
This effect can be gained for spell making via spells, choosing the Atronach birthsign, or the Atronach Stone Doomstone.

See also
Sections:
Reflect vs Absorb vs Resist: Information on the similarities and differences between these effects. 

Related effects:
Absorb Magicka, Fortify Magicka, Restore Magicka
Reflect Spell, Resist Magic, Weakness to Magic