Additionally, a spellcaster loses one spell or spell slot from his or her highest available level. Negative levels stack.
frankthedm said:I believe the "per day"
If the victim casts spells, she loses access to one spell as if she had cast her highest-level, currently available spell. (If she has more than one spell at her highest level, she chooses which she loses.) In addition, when she next prepares spells or regains spell slots, she gets one less spell slot at her highest spell level.
LEVEL LOSS
A character who loses a level instantly loses one Hit Die. The character’s base attack bonus, base saving throw bonuses, and special class abilities are now reduced to the new, lower level. Likewise, the character loses any ability score gain, skill ranks, and any feat associated with the level (if applicable). If the exact ability score or skill ranks increased from a level now lost is unknown (or the player has forgotten), lose 1 point from the highest ability score or ranks from the highest-ranked skills. If a familiar or companion creature has abilities tied to a character who has lost a level, the creature’s abilities are adjusted to fit the character’s new level.
The victim’s experience point total is immediately set to the midpoint of the previous level.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.