In D&D 3rd Edition, a class of saving throws that are based on character's ability to avoid an attack by dodging out of the way. Reflex saves are used for things like jumping out of the way of a trap and avoiding effects like web and entangle spells. Also, many area-effect spells (fireball, lightning bolt, and their numerous kin) allow a reflex save for half damage. A character's attempt to dodge out of the way of ray spells like Disintegrate and Ray of Enfeeblement is not done with a saving throw in 3rd Edition; rather, the attacker makes a ranged touch attack with the ray.
Your reflex save is modified by your dexterity bonus.