Term: Difficult Terrain

Difficult Terrain is a kind of environmental hazard that makes Movement difficult. It costs 2 ft to move through 1 ft of difficult terrain (or doubling the Movement cost).
Low furniture, rubble, undergrowth, steep stairs, snow, and shallow bogs are examples of difficult terrain. Flying without any assistance (even with a Flight speed) is considered moving through Difficult Terrain.
The full rules for this can be found at Rule: Difficult Terrains