SRD:Evasion and Improved Evasion

Evasion
These extraordinary abilities allow the target of an area attack to leap or twist out of the way. Rogues and monks have evasion and improved evasion as class features, but certain other creatures have these abilities, too.

If subjected to an attack that allows a Reflex save for half damage, a character with evasion takes no damage on a successful save.

As with a Reflex save for any creature, a character must have room to move in order to evade. A bound character or one squeezing through an area cannot use evasion.

As with a Reflex save for any creature, evasion is a reflexive ability. The character need not know that the attack is coming to use evasion.

Rogues and monks cannot use evasion in medium or heavy armor. Some creatures with the evasion ability as an innate quality do not have this limitation.

Improved Evasion
Improved evasion is like evasion, except that even on a failed saving throw the character takes only half damage.