The caster level of a cast spell is a measure of the power of that particular casting. For spells cast by a character, this is the character's level in the class used to cast the spell. For spells cast from items, the caster level is given in parentheses after the relevant item property. Epic spells cast from items are an exception in that their caster level is not given in parentheses. The caster level for most default epic spell item properties is 25, with the exceptions being epic warding and hellball, for which the caster level is 30. Sequencers (and other items that allow the dynamic setting of spells to be cast) are another exception, as they use "cheat casting" which uses a caster level of 2×innate level − 1, with a minimum of 10.
There are two general game mechanics that make use of caster level: dispelling and overcoming spell resistance. In addition, many—but not all—spells base the magnitude of their effects on caster level. (Cantrips are a notable exception.) The details vary by spell; commonly, either the duration or the number of dice rolled for damage is based on caster level.
Level drain affects caster level, reducing the power of all spells cast while that effect is present.
Note: A common misconception is that pale master levels add to caster level. They do not.
The caster level is set at the moment of casting any spell or special ability (before the casting animations). Spells cast from items will restore the caster level to its previous value (if it was previously higher than the caster level the item used) after the spell script has fired. Persistent spells (those that need to reference caster level after some time has passed) may use an incorrect caster level as they can trigger without resetting the caster level. This may cause unusual fluctuations in damage (as in blade barrier) if the caster makes use of different spells or special abilities thereafter.
Special abilities can also have caster levels associated with them. For player characters, these special abilities usually come in the form of feats associated with a particular class; the caster level of these special abilities is the associated class level. (This is not affected by level drain.) Another common source of special abilities is the result of a polymorph; when the ability is a spell, the caster level is the level of the first class taken. For monsters, special abilities are usually assigned in the Toolset; at the same time, each such ability is assigned a fixed caster level (possibly unrelated to any class level). Other monster special abilities are implemented with "cheat casting" (a scripted casting of the spell without needing access to it), which uses a caster level of 2×innate level − 1, with a minimum of 10.