户外 (hù wài) refers to the outdoors or open air, especially as a setting for activities (e.g., 户外运动 'outdoor sports'). It is more specific than 外面 (wài miàn, 'outside'), which just means outside a building, and less wild than 野外 (yě wài, 'wilderness'). Unlike 露天 (lù tiān, 'open-air'), which often describes a venue or structure, 户外 is a general noun for the outdoor environment.