Caster level

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 had a caster level equal to class level - 3, to reflect that they cannot cast spells before level 4. 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).