Oblivion:Absorb Magicka

Transfer M points of Magicka from the target to the spellcaster.  On the target, the effect is comparable to Damage Magicka; on the spellcaster the effect is comparable to Restore Magicka.  

Absorb Magicka in Spells
Absorb Magicka is not a very effective way to regenerate magicka when used in purchased or custom spells. It is better to think of such spells as providing a cheap version of Damage Magicka. 
For example, the standard Absorb Magicka spell has a base cost of 14 magicka, but only regenerates 10 magicka, resulting in a net loss of 4 magicka (for 33 Restoration).  
Your Restoration skill must be at least 58 to not lose magicka when casting the spell (and even then, only if your spell effectiveness is 100% and the enemy has 10 magicka for you to absorb).
At the Spell Making Altar, creating a custom spell with lower magnitude and longer duration will result in a greater net gain in magicka, but the spell might break early if the enemy moves too far away. 

Absorb Magicka is much more useful for replenishing magicka when cast using an Absorb Magicka scroll, because you don't expend any magicka while you absorb the target's magicka.
 Absorb Magicka scrolls provide a steady transfer of magicka over time. These scrolls are useful for giving you the advantage over an enemy in an extended magical battle.

Only "On Touch" absorb spells are possible, even at the Spellmaking Altar.

When using Absorb Magicka spells to regenerate magicka, be wary of enemies with Reflect Spell, because they may end up fully depleting your magicka reserves.

Absorb Magicka Enchantments
Enchanted weapons are a very effective way to use Absorb Magicka. 
There are various generic weapons enchanted with Absorb Magicka that can be found in random loot.
The enchanting altar can be used to create custom Absorb Magicka weapons.
A Sigil Stone provides the most useful Absorb Magicka effect by far. With up to 50 pts of absorption per strike, a weapon enchanted this way can quickly render a magical opponent powerless while providing the player with an abundance of magicka to use in the course of battle.
Despite the availability of Absorb Magicka for custom enchanting on Bows, the range limitation inherent in Absorb effects makes the effect largely useless. The Marksman must be very close for the Absorb effect to apply at all, thereby defeating the purpose of using a Bow. The maximum range is currently unknown. There are no patches known which address this issue.

Notes
This effect can be gained for spell making and enchanting via spells only.
This effect will stop as soon as the target reaches 0 pts magicka. So it won't even start to work on many creatures with a magicka pool of 0 pts. But you can cast a fortify magicka (3 pts for 120s is already enough) on them first, then use a custom spell with fortify magicka + absorb magicka + weakness to magic, to infinitely absorb magicka from any enemy, before the first fortify magicka effect expires.

See also
Sections:
Drain vs. Damage vs. Absorb: A comparison of these similar effects.
Restore vs. Fortify vs. Absorb: A comparison of these similar effects.

Related effects:
Damage Magicka, Drain Magicka, Fortify Magicka, Restore Magicka
Absorb Attribute, Absorb Fatigue, Absorb Health, Absorb Skill