Uneven Ground
Source Player Core pg. 415
Uneven ground is an area unsteady enough that you need to Balance or risk falling prone and possibly injuring yourself, depending on the specifics of the uneven ground. You're off-guard on uneven ground. Each time you are hit by an attack or fail a save on uneven ground, you must succeed at a Reflex save (with the same DC as the Acrobatics check to Balance) or fall prone.