Caster level

From DDO wiki

Your Caster level is the level in your casting class, thus it may be different from your character level in the case of a multiclass character.

Note that in PnP, a Paladin or Ranger has a caster level equal to class level divided by two (but zero until level four, since they cannot cast spells before that). DDO does not follow that reasoning, so Paladins and Rangers start at caster level 4 immediately once they gain any spells (i.e. when they reach level 4).