Difficult Terrain

Combat rarely takes place in bare rooms or on featureless plains. Boulder-strewn caverns, briar-choked forests, treacherous staircases--the setting of a typical fight contains difficult terrain.
Every foot of movement in difficult terrain costs 1 extra foot. This rule is true even if multiple things in a space count as difficult terrain.
Low furniture, rubble, undergrowth, steep stairs, snow, and shallow bogs are examples of difficult terrain. The space of another creature, whether hostile or not, also counts as difficult terrain.