Spell Focus

(Player's Handbook v.3.5, page 100, DnD 3.5)

[General]

Choose a school of magic, such as illusion. Your spells of that school are more potent than normal.

Benefit

Add +1 to the Difficulty Class for all saving throws against spells from the school of magic you select.

Special

You can gain this feat multiple times. Its effects do not stack. Each time you take the feat, it applies to a new school of magic.