Barbarian class feature. Earliest BAB +1. Earliest improvement BAB +6.
Sometimes stacks with uncanny dodge (if the character already has uncanny dodge, gaining uncanny dodge again upgrades it to improved uncanny dodge): assassin, barbarian, dwarven defender, fist of the forest, rogue, shadowdancer, streetfighter. Sometimes this does not happen: scout.
Always at least a two-level gap in gaining improved uncanny dodge after uncanny dodge (unless there’s a way to do it using prestige classes).
Tied to base attack bonus because it is an ability derived from combat experience. Classes that don’t have a good Reflex save still gain this class feature.