Caster level

From BRWiki

A caster level is generally equal to the number of class levels in a spellcasting class. Some prestige classes add caster levels to an existing class.

As a caster's level increases, so does the power and number of spells that they can cast as well as which spells they can have access to. For spellcasters who are regents, the caster level also controls what realm spells they can cast.

See Also: Spell level

This is partially quoting from the D20 SRD.